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GEN CHEM FLSHCRDS
Terms in this set (33)
A researcher experimentally determines the atomic mass of an iron sample to be 55.1 amu. Which of the following most likely explains this result?
A. The sample has undergone beta decay.
B. Part of the sample has been reduced.
C. The sample has tin contamination.
D. Part of the sample has been oxidized to Fe2O3.
Atomic mass values listed on the periodic table are weighted averages of the naturally occurring isotopes for each element. 55.1 amu is lower than expected. Fe is metallic and is much more likely to be oxidized than reduced. Beta decay (β-) results in the conversion of a proton to a neutron and an emitted beta particle. Atomic mass does not change significantly in beta decay. Contamination with tin, which is significantly heavier than iron, would yield a higher than expected atomic mass. This leaves the correct answer primarily by process of elimination.
When iron is oxidized it forms rust (Fe2O3) that contains oxygen, which has lower atomic mass than iron
*. Rust contamination would therefore give lower atomic mass than expected.
Which of the following aqueous mixtures will produce a Cl- concentration of 2 M at 25°C?
A. 271 g of HgCl2 in 55 mol of H2O
B. 53 g of NH4Cl in 22.5 mol of H2O Correct Answer (Blank)
C. 271 g of HgCl2 in 22.5 mol of H2O
D. 53 g of NH4Cl in 55 mol of H2O
53 g of NH4Cl in 22.5 mol of H2O
has a Cl- concentration of 2 M at 25°C. Pure water has a concentration of 55 M and density of 1 kg/L. 55 moles of water represent 1 L (1 kg) and 22.5 moles of water represent 0.5 L (0.5 kg). HgCl2 demonstrates poor water solubility and will dissociate little. Therefore, 1 mol of HgCl2 placed in either water 1 kg or 0.5 kg of water will not achieve a 2 M Cl- solution, eliminating choices with "HgCl2." NH4Cl is water soluble and has a molecular weight of 53 g/mol. "53 g of NH4Cl in 55 mol of H2O" is a 1 M NH4Cl (1 M Cl-) solution and "53 g of NH4Cl in 22.5 mol of H2O" is a 2 M NH4Cl (2 M Cl-) solution.
Consider the unbalanced equation NaHCO3(s) --> Na2CO3(s) + CO2(g) + H2O(g). How many grams of water will be produced by 1 mole of sodium bicarbonate?
The balanced equation is
2 NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(g).
Therefore, 1 mole of NaHCO3 will produce ½ mole of H2O, i.e., ½(2 + 16) = 9 grams of H2O.
When heated, tungsten(VI) oxide, WO3, reacts with hydrogen gas to produce metallic tungsten and water vapor. Assuming that the reaction goes to completion, how many moles of solid tungsten are produced for every mole of hydrogen gas consumed?
When heated, tungsten(VI) oxide, WO3, reacts with hydrogen gas to produce metallic tungsten and water vapor. Assuming that the reaction goes to completion, 1/3 mole of solid tungsten are produced for every mole of hydrogen gas consumed.
Since the balanced equation for the reaction is WO3 + 3 H2 W + 3 H2O,
the ratio of W produced to H2 consumed is 1/3.
What is the total mass of the hydrogen atoms contained in 3 moles of glucose (C6H12O6)?
The mass of one mole of hydrogen atoms is 1 g. Glucose contains 12 H atoms, so one mole of glucose contains 12 g of hydrogen. Three moles of glucose would therefore contain 3(12 g) = 36 g of hydrogen.
A compound is found to contain 64% silver, 8% nitrogen, and 28% oxygen by mass. What is the empirical formula of this compound?
For purposes of calculation, assume that we have 100 g of the compound. This would mean that we have 64 g of Ag, 8 g of N, and 28 g of O. Therefore, the number of moles of each of these elements is:
Ag: (64 g) / (108 g/mol) = 0.6 mol
N: (8 g) / (14 g/mol) = 0.6 mol
O: (28 g) / (16 g/mol) = 1.8 mol
Since the ratio of Ag:N:O is 1:1:3, the empirical formula must be
Cu2S --> 2 Cu + S
If two moles of Cu2S are consumed in the reaction above, how many moles of solid copper are produced?
f two moles of Cu2S are consumed in the reaction above,
4 moles of solid copper are produced.
If each mole of Cu2S produces 2 moles of Cu, then two moles of Cu2S will produce 4 moles of Cu.
2 Ag2O --> 4 Ag + O2
If one mole of Ag is produced in the reaction above, how many grams of oxygen gas are produced?
If one mole of Ag is produced in the reaction above, 8 g of oxygen gas are produced.
According to the given balanced equation, when 4 moles of Ag are produced, 1 mole of O2 is produced. Therefore, if 1 mole of Ag is produced, then 1/4 mole of O2 will be produced. The mass of 1/4 mole of O2 is (1/4)(2)(16 g) =
Why do chlorine atoms form anions more easily than they form cations?
Chlorine atoms form anions more easily than they form cations
because chlorine can gain one electron in order to complete its outer shell.
The quest for closed-shell stability is the primary driving force in chemical processes. When chlorine gains an electron, it completes its octet, so chlorine atoms form anions much more easily than they form cations.
What is the molar concentration of a solution containing 0.5 mol of solute in 50 cm3 of solution?
By definition, molarity equals moles of solute divided by liters of solution. Since 1 cm3 = 1 mL, the molarity of this solution is (0.5 mol)/(0.05 L) =
What is the concentration of I- ions in a 0.20 M solution of magnesium iodide?
Since the empirical formula for magnesium iodide is MgI2, two moles of dissolved I- result from each mole of dissolved MgI2. Therefore, if [MgI2] = 0.20 M, then [I-] = 2(0.20 M) =
What is the molar concentration of Ca2+(aq) in a solution that is prepared by mixing 15 mL of a 0.02 M CaCl2(aq) solution with 10 mL of a 0.04 M CaSO4(aq) solution?
For dilution problems, like this one, the concentration (C) times the volume (V) of all the solutions before mixing must equal the concentration times the volume of the final solution:
(CV)Soln 1 + (CV)Soln 2 = (CV)Final soln
(0.02M)(15 mL) + (0.04M)(10 mL) = C(25 mL)
0.028M = C
4 NH3 + 6 NO --> 5 N2 + 6 H2O
If 85 g of NH3 and 210 g of NO were used in the reaction above, which of the following would be the limiting reagent?
If 85 g of NH3 and 210 g of NO were used in the reaction above, NO would be the limiting reagent.
85 g of NH3 is equivalent to: 5mol of NH3
210 g of NO is equivalent to: 7mol of NO
According to the reaction, NH3 and NO react in a 4:6 molar ratio. In order for 5 moles of NH3 to react completely, there would have to be: 7.5mol of NO
available to react.
As there are only 7 moles of NO,
NO is the limiting reagent.
Why does the number of electrons not play a role in determining the mass number of an atom?
Mass number is defined as the number of protons and neutrons in a nucleus. Electrons are excluded from the determination of mass number since they are much less massive (1/1836 AMU) than a proton or neutron (both approximately 1 AMU),
Which of the following compounds would exhibit the strongest intermolecular forces?
CH3CO2H (acetic acid) possesses both a hydrogen bound to an oxygen and unbound electrons on another oxygen allowing it to form hydrogen bonds. Since hydrogen bonds are the strongest type of intermolecular force, this is the correct answer. CO2 is capable of only weak London dispersion forces given its linear geometry. C6H14 (hexane) is a nonpolar molecule, making it capable of only London dispersion forces as well (choices A and D are incorrect). CHCl3 can form dipole-dipole interactions which are stronger than dispersion forces, but not as strong as hydrogen bonds (eliminate choice B).
As you move from left to right across the periodic table, the atomic radii of elements in the same period:
As you move left to right across a period, the elements have more protons, giving the atoms increasing effective nuclear charges after the shielding of the inner core electrons are factored in. This change in effective nuclear charge due to proton number plays a more significant role across a period than changes in shielding. The increased effective nuclear charge means that the valence electrons feel a stronger pull to the nucleus, thus the radius decreases from left to right. Any electron-electron repulsion is more than counteracted by the increased effective nuclear charge.
Compared to uncatalyzed reactions, the Ea - ΔHrxn for catalyzed reactions is:
lower because catalysts decrease Ea.
Given the same number of moles, which of the following solids will have the lowest melting point?
Hydrogen selenide is a molecular solid. Its intermolecular forces consist of dipole-dipole interactions. Though a relatively strong intermolecular force, they are much weaker than the covalent bonds between the atoms in a network solid (choice A), the covalent bonds between the atoms in a metallic solid (choice B), and the ionic forces between the ions in an ionic solid (choice C). Thus, the weaker intermolecular forces will allow hydrogen selenide to melt more easily than the others.
Which type of visible light, red or violet, has the highest energy?
High energy is synonymous with short wavelength and high frequency because E = hf and f = c/λ. This eliminates choices A and D. Violet light is adjacent to ultraviolet radiation (waves that are bad to the body) in the electromagnetic spectrum, while red light is adjacent to infrared radiation (waves that are benign to the body, also known as heat). Thus, violet must have higher energy, making choice B correct.
Which pair of liquids below will be immiscible?
A. H2O and CCl4
B. CH3OH and CH3CH2OH
C. CH3CH2CH2CH2CH3 and CH3CH2CH2CH2Br
D. CCl4 and C6H6
The solvent pair that is immiscible will be made of a polar and a nonpolar compound based on the rule of thumb that "like dissolves like." Therefore, compounds that have very different intermolecular forces are least likely to dissolve in each other, and will separate into two layers. Both methanol and ethanol can hydrogen bond, making choice B a miscible pair. Pentane is completely nonpolar, and bromobutane has only one slightly polar C-Br bond. They will both display London dispersion forces (LDFs) (perhaps some weak dipole-dipole forces for the alkyl halide) and be soluble in each other, eliminating choice C. Both CCl4 and C6H6 (benzene) are nonpolar compounds held together by LDFs, so are also soluble, eliminating choice D. Since water is a highly polar, hydrogen-bonding compound while carbon tetrachloride is nonpolar, choice A is the immiscible pair.
Which of the following species has/have a trigonal planar shape?
The shape of a molecule is determined by the total number of electron groups around the central atom, which determines the angles between the groups, as well as the number of atoms bonded to the central atom. PF3 has four electron groups around the central phosphorus (three single bonds to fluorine and a lone electron pair). It therefore has a trigonal pyramidal shape. Based on the remaining choices, BCl3 must be trigonal planar; it has three electron groups surrounding the central boron (three single bonds to chlorine). The carbonate ion (CO32-) has three electron groups around the central carbon (two single bonds to oxygen and one double bond to oxygen), making it trigonal planar as well.
Which of the following represents the ground state electron configuration of a Co(III) ion?
The ground state electron configuration of a neutral cobalt atom is [Ar] 4s2 3d7. Co(III) is a cation as metals do not gain electrons. Therefore, the electron configuration should have three fewer electrons than a neutral cobalt atom. Since the valence electrons of cobalt are in the fourth energy level, these are the first electrons lost. While choice D might look appealing since it has a half-filled d subshell, this type of electron configuration is only seen for neutral atoms in the chromium and copper families, so choice D can be eliminated. Both 4s valence electrons will be lost in addition to the then 3d valence electron, making choice C the best answer.
Which of the following pairs will have the strongest intermolecular forces between them?
A. O2 and N2
B. HCl and CHCl3
C. NaCl and NH3
D. H2O and C8H18
The strongest intermolecular forces occur between molecules with a greater magnitude of charge. Na+ and Cl- ions have full +1 and -1 charges, respectively. Polar compounds will have larger partial charges due to differences in electronegativity between the bonded atoms, while nonpolar compounds have little to no partial charge. Therefore, the ions in NaCl and the polar NH3 will form the strongest ion-dipole interactions. All other interactions will be much weaker, including the weakest instantaneous-induced dipole interactions between the nonpolar oxygen and nitrogen (choice A), dipole-dipole forces between the polar HCl and chloroform (choice B), and dipole-induced dipole interactions between the polar water and nonpolar octane (choice D).
In electrochemistry, which of the following best demonstrates the first law of thermodynamics?
The first law of thermodynamics states that energy is always conserved, thus the conversion of chemical energy into electrical energy is an example of the first law. A reaction proceeding spontaneously (resulting in an increase in overall entropy) is more a reflection of the second law of thermodynamics. Similarly, a reaction always proceeds to a probable outcome, hence why each reaction proceeds in the spontaneous direction.
Two excited state Bohr atoms experience electron transitions, the first atom E3 to E2, and the second atom from E2 to E1. The electron in the first atom loses:
a smaller energy, thus emitting a photon with a longer wavelength.
If an atom's atomic number equals its atomic weight, which of the following would be true?
The number of protons equals the number of electrons.
What clearly defines the P and V terms of the van der Waals equation for real gases?
P is the pressure caused by collisions with the walls of the container, and V is the volume of free space in the container.
Carbon monoxide gas reacts with oxygen to yield carbon dioxide, according to the balanced equation below. If 2.5 mol of O2 reacts with an excess of CO, what is the maximum volume of CO2 that can be produced at 760 mm Hg and 25°C? (R = 0.0821 L·atm/K·mol)
2CO + O2 ---> 2CO2
First, address the stoichiometry of the reaction. For every mole of O2 used, 2 moles of CO2 are produced. Therefore, the question asks for the volume of 5.0 mol CO2 at 1 atm (760 torr) and 298 K (25°C). Use the ideal gas law (the given gas constant R was a big hint) and do some MCAT math approximating, paying attention to the necessary units shown above.
PV = nRT or V = nRT / P
V = (5 mol)(~0.08 L·atm/K·mol)(~300 K) / (1 atm)
V ≈ 120 L
A sample of helium gas at 300°C and 600 torr is in a 200 L container. If the temperature of the gas is raised to 900°C at constant volume, how is the pressure of the gas affected?
Pressure will increase by a factor of 2 since the temperature increases by 600°C.
Which gas will effuse 4.5 times more slowly than He in an effusion experiment?
Kr will effuse 4.5 times more slowly than He in an effusion experiment. According to Graham's law, the ratio of the rates of effusion of two gaseous molecules is equal to the square root of the inverse ratio of their molecular weights:
Squaring both sides of this equation gives a molecular weight of the unknown as approximately 81, making Kr the best answer.
A 44.8 L container is filled with 1 mole of ideal gas X and 1 mole of real gas Y at 0°C. The pressure inside the container is:
A 44.8 L container is filled with 1 mole of ideal gas X and 1 mole of real gas Y at 0°C. The pressure inside the container is between 0.5 atm and 1 atm.
The volume of one mole of an ideal gas at STP is defined as 22.4 L. If both gases were ideal in this 44.8 L container, the 2 moles would give a combined pressure of:
However, since gas Y is a real gas, its molecules experience intermolecular forces resulting in less force exerted on the walls of the container. The partial pressure exerted by the real gas is less than its ideal partial pressure. Therefore, the total pressure is 0.5 atm from ideal gas X plus something less than 0.5 atm from real gas Y, making the total pressure between 0.5 atm and 1 atm.
Which of the following is NOT a criterion for a gas to be considered ideal?
A. The molecules have negligible volume.
B. The molecules have negligible mass.
C. The molecules do not experience intermolecular forces.
D. The molecules do not liquefy at low temperature.
B. The molecules have negligible mass.
A confined gas at 0°C occupies a volume of 55.6 L. Determine the volume of this quantity of gas if the pressure is held constant but the temperature is lowered to -10°C
A confined gas at 0°C occupies a volume of 55.6 L. The volume of this quantity of gas will be 53.6 L if the pressure is held constant but the temperature is lowered to -10°C.
At constant pressure, the volume drops by a factor of 1/273 for every 1 kelvin (or 1°C) drop in temperature. Thus, the volume here drops by (10 / 273) × (55.6 L) = 2 L, so the final volume is 55.6 - 2 = 53.6 L.
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