Homework questions.

The mass number of an atom is 15, and its atomic number is 7. The atom probably has...

8 neutrons in the nucleus because the mass number is the number of protons plus neutrons.

Which statement is true of atoms? Electrons determine the atom's size. Protons repel electrons. Protons attract other protons. Most of an atom's volume is filled with matter. All of the above.

Electrons determine the atom's size. With electrons on the surface, atoms repel one another when they come too close. Thus, electrons determine the space that an atom occupies.

Dr. Jones says an atom has 3 electrons in the first shell and four electrons in the second shell. Someone should tell Dr. Jones that ... the second shell can't have 4 electrons. the first shell shouldn't have 3 electrons. the second shell should have 8 electrons. no shell can hold more than 2 electrons. the first shell must fill before the second shell can have electrons.

the first shell shouldn't have 3 electrons.

Which statement is true of the energy levels of electrons in shells? Electrons must lose energy to move from the first to the second shell. The valence shell has higher energy than other occupied shells. All the electrons in an atom have similar amounts of energy. All of the above. None of the above.

The valence shell has higher energy than other occupied shells. The outer occupied shell is the valence shell, and outer shells hold electrons at higher energy than inner shells.

The atom has a vacancy in its inner shell; an electron has moved to the outer (valence) shell. The result is an unstable excited state. What will be the fate of the electron?

An electron will move back to the inner shell, restoring the ground state as the atom gives off energy.

An orbital is dumbbell-shaped. Which statement is most likely true? Each lobe can hold one electron. Its electrons move along a figure-8 path. It's in the first electron shell. Each lobe can hold two electrons. None of the above.

None of the above because Only the second and higher shells have dumbbell-shaped orbitals. Each orbital can hold just two electrons, and the electrons move in unknown paths such that both electrons can be in one lobe at one moment, and in opposite lobes at the next moment.

Two atoms always represent the same element if they have ... the same number of electrons. the same mass number. the same number of shells. the same number of protons. the same number of particles in the nucleus.

the same number of protons.

An atom has 6 electrons, 6 protons, and 6 neutrons. You can tell that this atom belongs to the element _____ because _____________________.

C; it has 6 protons.

An atom has 8 protons, 8 neutrons, and 8 electrons. Another isotope of the same element might have ...

10 neutrons. Isotopes have the same number of protons; different numbers of neutrons.

Radioactive decay is likely to occur when ...

an atom has too many neutrons. The atomic nucleus becomes unstable if the ratio of protons to neutrons is too far from unity.

Which model most accurately represents the current view of the structure of the atom? Probability model Planetary model

Probability model because We cannot determine the exact motion of electrons. We can only compute the regions where electrons occur most of the time. To emphasize the uncertainty, a diagram showing random motion is more suitable than planetary paths.

Which statement is most useful in explaining why chemists assign atoms to chemical elements by counting protons? The nucleus doesn't change in stable isotopes. 99% of the atom's mass consists of protons. Protons at the atom's surface determine the atom's behavior. The proton's negative charge holds electrons in the atom. None of these. Elements are defined by the number of protons.

None of these. Elements are defined by the number of protons. Protons give an enduring basis for atomic behavior because they are permanently anchored in the nucleus, where their attraction determines how many electrons the atom can hold. By contrast, electrons come and go, and the numbers vary.

Covalent bonds hold atoms together because they ... (a) fill shells without giving atoms much charge. (b) bring electrons closer to protons. (c) use forces between nuclei as well as forces between electrons. do all of the above. do both (a) and (b).

do both (a) and (b). Electron-sharing brings electrons closer to nuclei and it fills shells without giving the atoms much charge. Both factors stabilize the bond.

4, 1, 2, 3.

An atom's atomic number is 7. Its valence is most likely ...

3 because The neutral atom has 7 electrons. Two electrons fill the first shell, and 5 go into the second (valence) shell. The atom needs 3 more electrons to fill the valence shell.

By making two covalent bonds, an O atom (with 8 protons) fills its valence shell. Why does the atom's charge stay close to zero?

Shared electrons aren't always near oxygen because Oxygen keeps 6 electrons to itself. The 4 shared electrons migrate between O and the other atoms, thus contributing 4 half-charges to oxygen. This gives oxygen about 8 units of negative charge, balancing the atom's 8 protons.

In a double covalent bond, a carbon atom shares ...

electrons in two orbitals. A double bond involves two orbitals, each with a shared pair of electrons.

The ammonia molecule in the diagram has the observed bond orientation because ... N has 7 protons in its nucleus. electrons repel one another. N has four pairs of electrons in the valence shell. All of the above. None of the above.

All of the above. Since N has 7 protons, it must fill the second shell, giving it 4 pairs of electrons. The electrons form 3 bonds and 1 lone pair of electrons. Each pair of electrons repels the other pairs, so they are equally far apart.

Without making or breaking bonds, the pictured molecule can change its shape because ...

rotation can occur around single bonds. Rotation allows groups of atoms to change position while preserving the usual bond angles around each atom.

Two C atoms form a double bond. Each C is bound to two H atoms. Which statement is true? All the atoms lie in a plane. The bonds orient in a tetrahedral fashion. The bonds orient like tripods or pyramids. The groups rotate around the C=C bond. All the atoms lie in a line.

All the atoms lie in a plane. The double bond limits all the atoms to a plane.

Partial charges occur when ... (a) a covalent bond links atoms of two kinds. (b) atoms share electrons unequally. (c) two ions are close together. any of the above occur. both (a) and (b).

(b) atoms share electrons unequally. Each kind of atom has a different attraction for electrons. Thus, atoms of different kinds share electrons unequally. That gives one atom a partial negative charge; the other atom a partial positive charge.

To fill the valence shell, an electrically neutral, unbonded atom with atomic number 8 must add ...

2 electrons because The neutral atom has 8 electrons. Two electrons fill the first shell, and 6 go into the second (valence) shell. Two more electrons would fill the valence shell.

Which answer helps to explain why carbon atoms tend to make 4 covalent bonds? The first electron shell has 4 orbitals. The valence shell needs 8 electrons. All of the above. None of the above; carbon makes 3 covalent bonds.

The carbon nucleus has 4 protons. Without help, C can only hold 4 electrons in the valence shell. To fill the shell, 4 more electrons must be shared, making 4 covalent bonds.

1

O, N, C, H

Which atom in the pictured molecule will have the strongest partial positive charge? The O atom that's in C=O. The C that's in C=O. The N atom. The H that's bound to O. The C that's bound to N.

The C that's in C=O.This C atom is bound to the two most electronegative atoms in the molecule (O), both of which are pulling electrons from C.

Dr. Haxton says the O-O bond is polar and the C-C bond is nonpolar. A good student would say ... Right! O is electronegative, so O2 is polar. Wrong again, Ralph. Both bonds are nonpolar. Yes. O attracts electrons more strongly than C. No, both bonds are highly polar. No way. C is more electronegative than O.

Wrong again, Ralph. Both bonds are nonpolar. When two atoms of the same kind form a covalent bond, they share electrons equally because their electronegativity is the same.

Use your knowledge of electronegativity to pick out the most polar bond in the list below. C-H O-N O-H N-H O-C

O-H. O and H are at opposite ends of the electronegativity scale, so the polarity is very large.

If you want a molecule that is highly polar, look for one that contains:

H-N. N and H differ in electronegativity more than any other bonded pair of atoms in the list, making the N-H bond most polar.

Which item is based on differences in electronegativity between atoms? (a) The organization of phospholipids into bilayers. (b) The pairing of bases in DNA. (c) The tertiary folding of proteins. Both (a) and (b). All of the above.

All of the above.All these processes depend on polarity and hydrogen bonds, which are based on differences in electronegativity.

Compared to covalent bonds, ionic bonds ..

rely much less on electron-sharing. Covalent bonds are entirely based on sharing electrons, whereas ionic bonds are attractions between ions that keep their electrons.

A cation with two units of charge has 10 neutrons and 8 protons. The ion also has ...

6 electrons. The part with the two units of + charge is a cation; the part with the unit of - charge is an anion.

Which statement is true of the molecule shown here? n here?

It's both a cation and an anion. The part with the full unit of + charge is a cation; the part with the unit of - charge is an anion.

The atoms shown here will become ions if they meet because ...

The valence shell is unfilled in both atoms. By taking an electron from lithium, fluorine can fill its valence shell. When lithium loses the electron, its outer shell is emptied; its full inner shell becomes its valence shell. Both atoms become stable ions.atoms tend to have filled valence shells.

True or false: Ions may form by transferring an atomic nucleus (with or without electrons) to another molecule.

T; For example, a hydrogen nucleus (H+) may move between two water molecules, making an H3O+ ion and an OH- ion.

A phrase that applies to covalent bonding and not other kinds of bonds is ...

electron-sharing. Only in covalent bonds do atoms share electrons.

When chemists and biologists want to show how atoms are bonded in a molecule, they usually use a ...

structural formula.

Polar attractions are ... (a) forces between atoms with partial charges. (b) weaker than covalent bonds. (c) important because they are numerous. (a) and (b) are true. (c) is false. (a), (b), and (c) are correct.

(a), (b), and (c) are correct;Polar attractions occur between atoms with partial charges and are weaker than covalent bonds. Still, they're important because so many of them occur in living cells.

Hydrogen bonds ... are covalent bonds. share electrons between the two bonded atoms. can form between H and N. occur within a water molecule. All of the above.

can form between H and N; They can also form between H and other highly electronegative atoms, such as O, if the atoms have strong partial charges.

Which statement about the following reaction is true? 4 Fe + 3 O2 --.> 2 Fe2O3 The reaction is not balanced. Four grams of Fe reacts with 3 grams of O2 to form 2 grams of Fe2O3. Fe2O3 is the reactant in this reaction. Four atoms of Fe react with 3 molecules of O2 to form 2 molecules of Fe2O3 None of the above.

Four atoms of Fe react with 3 molecules of O2 to form 2 molecules of Fe2O3. You identified the reactants as Fe and O and the product as Fe2O3. You also knew that the coefficients in the equation correspond to the number of atoms or molecules that take part in the reaction

Choose the appropriate balanced equation for the following chemical reaction. C6H6 + H2 --> C6H12

C6H6 + 3 H2 --> C6H12;

Choose the correct balanced equation for the following reaction. C8H18 + O2 --> CO2 + H2O

2 C8H18 + 25 O2 --> 16 CO2 + 18 H2O

Which of the following is a chemical reaction? Changing a carbon atom to a nitrogen atom by radioactive decay. Melting ice. Dissolving a lump of sugar in a cup of coffee. Making a hydrogen bond between a water molecule and a sugar molecule. None of the above.

None of the above.; A chemical reaction alters the covalent bonding between atoms, making new kinds of molecules. That doesn't happen in any of the events listed here

Potential energy is always ...

associated with position or arrangement. ;Potential energy occurs in many forms, but it's always associated with the arrangement of objects in space. Often the objects exert forces on each other.

Which of the following is true of activation energy? (a) Molecules usually get it from thermal energy. (b) Molecules usually get it by breaking bonds. (c) It's the energy difference between reactants and products. Both (a) and (c). (a), (b), and (c).

(a) Molecules usually get it from thermal energy.; In a collision, thermal energy is converted to potential energy.

Which fact is most important in explaining how molecules convert thermal energy to activation energy (EA)?

Distortion increases a molecule's potential energy. ; In a collision, kinetic energy becomes the potential energy of distortion, which can weaken bonds enough to permit a reaction.

Which statement is true of the transition state? (a) It can break apart to yield either reactants or products. (b) It is extremely unstable and lasts only an instant. (c) It has less potential energy than the reactants but more than the products. Both (a) and (b). Both (b) and (c).

Both (a) and (b). ; It's an arrangement that quickly breaks apart, emitting either the original molecules or new product molecules.

Dr. Haxton says he has found a reaction that is truly irreversible. What would a good student say? What's new? We've always known that some reactions are truly irreversible. Patent it, Doctor. You can use it to make a perpetual motion machine. All reactions are irreversible. This is possible, given a suitable catalyst. Sorry, Doctor. All reactions are reversible.

Sorry, Doctor. All reactions are reversible. ; Some reactions look irreversible because the reverse reaction is very slow, but the laws of thermodynamics guarantee that all are reversible.

Which statement is true of the relation between life and chemical equilibrium? (a) Living cells spend energy to stay as close to chemical equilibrium as possible. (b) Living cells avoid chemical equilibrium by taking in energy and materials. (c) Life persists only if cells can stay near chemical equilibrium. Both (a) and (c). None of the above.

(b) Living cells avoid chemical equilibrium by taking in energy and materials. The only way to avoid equilibrium is to exchange materials and energy with the environment.

The reaction shown below is at equilibrium. What could you do to make this reaction produce more methyl benzoate? (a) Add more benzoic acid. (b) Remove some methanol (HOCH3). (c) Remove some water. (d) Add more water. Both (a) and (c).

Both (a) and (c);By removing water, you slow the reverse reaction; by adding a reactant, you speed the forward reaction. Both changes make the forward reaction faster than the reverse reaction. Therefore, the reaction will produce more methyl benzoate as it returns to equilibrium.

In the equation deltaG = deltaH - TdeltaS, the change in the amount of disorder in the system is symbolized by ...

deltaS; S is entropy, which is a measure of disorder.deltaS indicates how much the entropy is changed.

A chemist calculates that a certain reaction would result in deltaH = -9 kJ and TdeltaS = +1 kJ. What can you conclude?

The reaction will occur spontaneously, because it decreases G. ; deltaG = deltaH - TdeltaS = -9 -(-1) = -8. Since the free energy decreases, the reaction will run spontaneously.

For the reaction ATP + H2O \rightarrow ADP + Pi, the change in standard free energy (delta/circ) is -30.5 kJ/mol. From this information we can conclude that ... (a) the forward reaction will be spontaneous under standard conditions. (b) the reverse reaction will be spontaneous under standard conditions. (c) the reaction will run rapidly. Both (a) and (c). Both (b) and (c).

(a) the forward reaction will be spontaneous under standard conditions. ;

Under standard conditions the reversible reaction A + B --> C + D has adeltaG\circ = -25 kJ/mol. What is deltaG when: T = 475 K, [A] = 0.5M, [B] = 0.2 M, [C] = 0.2 M, and [D] = 0.3 M?

-27 kJ/mol ; The nonstandard concentrations have lowered deltaG by 2 kJ.mol.

Which statement is true of the relationship between life and chemical equilibrium? (a) Living cells spend energy to stay as close to chemical equilibrium as possible. (b) Life depends on enzymes that move reactions away from equilibrium. (c) Life persists only if cells can stay near chemical equilibrium. (d) Living cells avoid chemical equilibrium by taking in energy and materials. Both (a) and (c).

(d) Living cells avoid chemical equilibrium by taking in energy and materials.
Both (a) and (c).; The only way to avoid equilibrium is to exchange materials and energy with the environment.

Catalysts speed reactions mainly by ... providing activation energy. altering deltaG to favor the forward reaction over the reverse one. lowering EA. raising the kinetic energy of the reactants. None of the above.

lowering EA.; Catalysts always lower EA, though they may have other effects as well. With a lower EA, a larger fraction of collisions can produce the transition state.

194.2

How many grams of water (H2O) are needed to react with 5.0 grams of ferric trichloride (FeCl3)? FeCl3 + 3 H2O --> Fe(OH)3 + 3 HCl

1.7 g H2O ; It would take 1.7 g of H2O to react with 5.0 grams of FeCl3.

How many grams of glucose would you need to make 1 liter of an aqueous 0.5M glucose solution? (Atomic masses: C = 12.01, H = 1.008, O = 16.00.)

90.08 grams of glucose

Dr. Haxton told his class that a water molecule can make 4 hydrogen bonds, all of them in the same plane as the three atoms. What would a good student reply?

Wrong! Two hydrogen bonds are not in the plane of the atoms.; The array of hydrogen bonds is three-dimensional.

Which statement is part of the explanation for water's high cohesion? (a) Oxygen has four valence orbitals. (b) Oxygen is much more electronegative than hydrogen. (c) Water can hydrogen-bond with other kinds of molecules. All of the above. Both (a) and (b).

Both (a) and (b). ; he electronegativity difference creates charges that make hydrogen bonds possible, and the four outer orbitals permit water to make four hydrogen bonds. The result is a three-dimensional network of bonds between water molecules.

Despite its cohesion, water can flow because ...

hydrogen bonds break and re-form rapidly. ; There are always enough hydrogen bonds to hold the water together, but the bonds break and re-form rapidly, allowing molecules to move relative to one another.

Which answer helps to explain how many hydrogen bonds a water molecule can form? Which answer helps to explain how many hydrogen bonds a water molecule can form? The angle between H-O bonds in water is 180\circ. Water can ionize, making hydronium and hydroxide ions. Oxygen's valence shell has four orbitals. Oxygen makes two covalent bonds. The oxygen atom has six protons.

Oxygen's valence shell has four orbitals. ; Each of those orbitals can engage in a hydrogen bond.

Which statement must be mentioned in explaining why amphipathic molecules line up at a water surface? Which statement must be mentioned in explaining why amphipathic molecules line up at a water surface? Polar groups repel water. Polar groups attract one another. Nonpolar groups attract one another. Nonpolar groups repel water. All of the above.

Polar groups attract one another. ; Polar water molecules pull together, holding the polar parts of the amphipathic molecules and squeezing out the nonpolar parts. That leaves the nonpolar parts waving in the air.

Dissolving is best described as ...

a mingling of molecules and/or ions. ; When sugar dissolves in water, the sugar molecules remain intact but mingle with water molecules. When salt (NaCl) dissolves, its Na+ and Cl- ions separately mingle with water.

Water is a source of ______________ for chemical reactions in cells. (a) hydrogen atoms (b) oxygen atoms (c) energy Both (a) and (b) (a), (b), and (c).

Both (a) and (b); Many reactions incorporate O and H from water into biological molecules. This happens when you digest starch and protein, for example

Which statement is true of water's tensile strength? Which statement is true of water's tensile strength? (a) It results from hydrogen bonding. (b) It helps to pull water through plants. (c) It involves both cohesion and adhesion. Both (a) and (b). (a), (b), and (c).

(a), (b), and (c). ; Because of hydrogen bonding, water coheres to itself and adheres to cell walls. That makes it possible to pull water through plants without breaking the water column.

Water has surface tension because ...

hydrogen bonds between surface water molecules resist being stretched. ; The hydrogen bonds between surface water molecules are normally slightly stretched. Like a stretched sheet of rubber, the surface tends to contract and resists being penetrated.

Which of the following helps most to explain why water has a high specific heat? Which of the following helps most to explain why water has a high specific heat? (a) A water molecule can make 4 hydrogen bonds. (b) The water molecule has exceptionally strong covalent bonds. (c) Water temperature is exceptionally sensitive to heat. Both (a) and (b). Both (b) and (c).

(a) A water molecule can make 4 hydrogen bonds. ; When you heat water, much of the heat is used to break hydrogen bonds. Only the remaining heat can increase molecular motion, raising the temperature.

Which factor is important in making it possible to cool yourself by sweating? Think carefully! Which factor is important in making it possible to cool yourself by sweating? Think carefully! (a) Molecules collide with varied angles and speeds. (b) Hydrogen bonds are relatively weak. (c) Water has more energy at the body surface. Both (a) and (b). (a), (b), and (c).

(a), (b), and (c). ; Random collisions allow some molecules to accumulate more energy than other molecules. The weakness of hydrogen bonds lets those molecules escape, leaving the cooler molecules behind.

Though you add heat, the temperature of boiling water remains constant because ...

it takes energy to break hydrogen bonds. ; At boiling, all the added heat is used to break hydrogen bonds. Free of the water mass, the departing steam carries away all the added energy, with none left over to raise the temperature.

Which statement helps to explain why ice is less dense than liquid water? Which statement helps to explain why ice is less dense than liquid water? (a) Water molecules make hydrogen bonds at definite angles. (b) Cold molecules move less than warm molecules. (c) Hydrogen bonds lengthen in the cold. All of the above. Both (a) and (b).

Both (a) and (b). ; The ice lattice has open spaces because of the angles at which hydrogen bonds form. Heat energy can break water molecules free of the lattice so they move into the openings.

The open spaces in water's crystal structure make it possible for ... The open spaces in water's crystal structure make it possible for ... (a) aquatic life to exist at the North Pole. (b) water to have a low boiling point. (c) life to occur in hot springs. Both (b) and (c). (a), (b), and (c).

(a) aquatic life to exist at the North Pole. ; The openings in the lattice make ice less dense than liquid water, so it floats. That causes water to freeze from the top down, so there's always liquid in which organisms can live.

Why doesn't oil mix with water?

(c) Polar molecules attract one another. ; Water molecules cling to one another and won't part to make room for uncharged (nonpolar) molecules. There's no repulsion.

Water does many things that aid life, but its most important role is to ... Water does many things that aid life, but its most important role is to ... increase the bulk of living matter. keep proteins folded. dissolve many solutes. provide atoms for chemical reactions. buffer temperature changes.

keep proteins folded. ; This is one of several organizing effects that make water a vital ingredient in living cells. Without it, proteins wouldn't function and cells would lose their boundary membranes. Life wouldn't exist.

A solution contains only glycerol and fructose. There is ten times as much glycerol as fructose. Therefore, ...

glycerol is the solvent. ; In any solution, the most abundant substance is the solvent

A solution is aqueous if ...

water is the solvent.

A 1 M solution contains ...

(a) one mole of solute per liter of solution.

The water molecule has a bent shape because ...

oxygen has two unbonded pairs of valence electrons. ; Repulsion between electrons governs the angles between bonds. The two pairs of unbonded electrons repel the two pairs of bonding electrons, creating the bend in the molecule.

Which statement is true of water? Which statement is true of water? (a) The O atom in water has a partial positive charge. (b) The H atoms in water have partial positive charges. (c) Its polarity results from hydrogen's high electronegativity. (d) About 50% of the average cell's mass consists of water. All of the above except for (a).

(b) The H atoms in water have partial positive charges. ; H is less electronegative than O, so the shared electrons spend most of their time near O.

A compound is an acid if it ...

donates H+ to water.

In water, every sulfuric acid molecule transfers H+ to water and becomes an HSO4 - ion. Some HSO4 - ions give off another H+ to form SO4 -2 ions. Which statement is true? H2SO4 --> H+ + HSO4 - <--> H+ + SO4 2- H2SO4 is a strong acid and HSO4 - is a weak acid. H2SO4 and HSO4 - are strong acids. Both H2SO4 and HSO4 - are weak acids. H2SO4 is an acid; HSO4 - is not an acid.

H2SO4 is a strong acid and HSO4 - is a weak acid. ; Every H2SO4 molecule donates H+, so H2SO4 is a strong acid. Not all molecules of HSO4 - donate H+, so HSO4 - is a weak acid.

In water, phosphoric acid dissociates according to the chemical reactions written below. Which of the following statements about the dissociation of phosphoric acid is true? H3PO4 = H2PO4- + H+ H2PO4 = HPO4 2- + H+ HPO4 2- = PO4 3- + H+ H3PO4, H2PO4 - and HPO4 2- are all weak acids. H2PO4 - is a strong acid and H3PO4 and HPO4 2- are weak acids. HPO4 2- is a strong acid and H3PO4 and H2PO4 - are weak acids. H3PO4 is a strong acid and H2PO4 - and HPO4 2- are weak acids. H3PO4, H2PO4 - and HPO4 2- are all strong acids.

H3PO4, H2PO4 - and HPO4 2- are all weak acids. ; All 3 do not dissociate completely, so they are all weak acids.

Ammonia reacts with water as shown below. Which statement best explains why ammonia is considered to be a base? Bases are compounds that remove H+ from solutions. Bases are defined as compounds that add OH- to the solution. NH4 + can donate H+ to hydronium. The reaction decreases the amount of water in the solution. None of the above. Ammonia is not a base.

Bases are compounds that remove H+ from solutions. ; n becoming NH4 +, ammonia takes on H+. Removing H+ from a solution is the hallmark of a base.

Which of the following can be considered bases? Which of the following can be considered bases? (a) Na2CO3 (b) KOH (c) NaCl Both (a) and (b). All of the above

Both (a) and (b). ; both sodium carbonate and potassium hydroxide are bases!

Compounds that release OH- are bases because ...

(a) OH- combines with H+ and removes it from solution. ; This behavior of OH- earns the title of "base" for compounds that release OH-.

To determine whether a base is weak or strong, ...

look for undissociated molecules of base. ; Strong bases dissociate completely in solution; weak bases don't.

Which of the following can be considered strong bases? Which of the following can be considered strong bases? NaOCl NaOH NaHCO3 NH3 All of the above.

NaOH; This is the only base that dissociates completely in water.

Which statement is true of pH buffers? Which statement is true of pH buffers? (a) They consist of strong acids and strong bases. (b) They keep the pH of the blood constant. (c) They consist of weak acids and weak bases. Both (a) and (b). Both (b) and (c).

(c) They consist of weak acids and weak bases. ; By using weak acids and bases, the buffer can absorb or give off H+, stabilizing the pH.

A buffer consists of undissociated acid (HA) and the ion made by dissociating the acid (A-). How does this system buffer a solution against decreases in pH?

) A- reacts with H+ to become HA. ; A- binds most of the added H+, so not much H+ is left to lower the pH.

Buffers work best when ...

about half of the buffer molecules are dissociated. ; That arrangement gives the least change in pH when either acid or base is added.

How does the way a buffer stabilizes pH during addition of acid differ from the way the same buffer stabilizes pH during addition of base?

It's the same reaction running backward or forward. ; The buffer accepts H+ in one case; it gives off H+ in the other case.

Which answer helps to explain why all living cells need pH buffers? Which answer helps to explain why all living cells need pH buffers? Hydrogen bonds only form at medium pH values. Amino acid side chains have many carboxyl and amino groups. ATP will not deliver energy if it is ionized. Nucleic acids must have positive charges to form double helices. All of the above

Amino acid side chains have many carboxyl and amino groups; When these side chains ionize, they exert forces that affect the protein's folding. Thus, if pH isn't stabilized, the proteins will change their folding and may not function properly. To see the role of carboxyl and amino groups, see the lesson on Ionic Forces in Tertiary Structure, in the Proteins section.

To make a buffer, you need to ... To make a buffer, you need to ... (a) combine equal amounts of a strong acid and a strong base in water. (b) put either a strong acid or a strong base in water. (c) adjust the pH to 7.0. (d) have a weak acid or a weak base half ionized in water. Both (a) and (c).

(d) have a weak acid or a weak base half ionized in water.; When half of the molecules are ionized, there are plenty of molecules that can either absorb H+ or give off H+ when the pH is disturbed from outside.

Which answer is a true statement about pH values? Which answer is a true statement about pH values? pH is important in life mainly because it affects the cohesion of water. A high pH corresponds to a high H+ concentration. Stomach juice has a high pH. Pure water has a pH of 0. None of the above.

None of the above.

What conditions must be met in order for an aqueous solution to be called "neutral"?

The concentrations of hydronium and hydroxide ions are equal; In a neutral solution the concentrations of hydroxide and hydronium ions must be equal.

Factory smoke has led to acid rain that lowered the pH of a lake to 5.0. What is the H+ concentration in the lake?

10- 5 M ; The pH is the negative exponent of the H+ concentration.

Your tank of swamp fish needs a pH of 5, and the pH is 7 at present. What should you do to the H+ concentration?

Raise it to 100 times its present value. ; Quick, do it! Each pH unit is a 10-fold change in H+ concentration.

In a washing machine, the detergent raised the pH to 9.0. The concentration of OH- in the solution is ...

10-5 M; If you divide 10-14 by 10-9 M (the H+ concentration at pH 9), you get 10-5 M, the OH- concentration.

Your tank of alkali fish needs a pH of 8, and the pH is 6 at present. What should you do to the H+ concentration?

Reduce it to 1/100 of its present value. ; Each pH unit is a 10-fold change in [H+].

A solution is very acidic if it ... A solution is very acidic if it ... (a) has a very low pH value. (b) has a high hydronium ion concentration. (c) has a very low hydronium ion concentration. Both (a) and (b). Both (a) and (c).

Both (a) and (b).; Very acidic solutions have high hydronium concentrations, and as a result, have low pH values

The dissociation of water ... The dissociation of water ... (a) releases free H+ ions into the solution. (b) is quickly reversed, giving neutral water molecules. (c) involves pairs of water molecules. Both (a) and (b). Both (b) and (c).

Both (b) and (c). ; Water dissociates when H+ jumps from one water molecule to another. The H+ quickly jumps back.

Which statement is true of the equation shown below? H2O = H+ + OH It is a standard shorthand for water dissociation. It is correct in showing H+as a product of water dissociation. It omits an important water molecule. It omits an important water molecule and it is a standard shorthand for water dissociation. All of the above. None of the above.

It omits an important water molecule and it is a standard shorthand for water dissociation.; Chemists use this shorthand equation for simplicity and to emphasize H+, but in reality an H+ jumps from one water molecule to another, making hydronium and OH-.

Example: