185 terms

# Chem Lab Exam

Experiments 1-6
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Purpose of an Exhaust Hood
It has a fan to exhaust fumes out of the hood and away from the user
When is an exhaust hood used?
When studying noxious, hazardous or flammable materials
Which is correct? Pouring acids into water or water into acids?
Pour acids into water
Why do you pour acids into water (not the other way around)?
Because the heat of solution will cause the water to boil and the acid to splatter
Which common reagents are highly flammable?
Alcohols, acetone and ether
What does SI stand for?
International System of Units
Unit for Mass or Weight
gram
Gram
1 cubic centimeter of water at 4*C and 760 mmHg
Mass
quantity of material
Weight
mass times gravitational force
Unit for Length
meter (m)
1 meter = __ cm = __ mm = __ in.
100, 1000, 39.37
Unit for Volume
Liter
Liter
volume of 1 kg of water at 4*C
Unit for Temperature
*C
Temperature
measures heat intensity
Conversion between C andF
(9/5)C = F - 32
Unit for Heat
calorie
Calorie
the amount of heat required to raise 1 gram of water 1*C
1 cal = ____ J
4.184
Density for Liquids
g/mL
Density for Gases
g/L
Density for Solids
g/cm^3
Density
mass / unit volume
Specific Gravity
sp gr = density of a substance / density of a reference substance
Femto-
10^-15
Pico-
10^-12
Nano-
10^-9
Micro-
10^-6
Milli-
10^-3
Centi-
10^-2
Deci-
10^-1
Kilo-
10^3
Mega-
10^6
Giga-
10^9
SI Unit for Length
meter
1 inch = ___ cm
2.54
SI Unit for Volume
cubic meter (m^3)
1 L = __ cm^3 = __ m^3 = __ qt
1000, 0.001, 1.06
SI Unit for Mass
kilogram
1 kg = __ g = ___ lb
1000, 2.205
1 lb = ____ g
453.6
SI Unit for Energy
Joule (J)
SI Unit for Temperature / Conversion to *C
Kelvin (K)
*C + 273.15
What is the purpose of a lab balance?
It is used to obtain the mass of various objects.
What are graduated cylinders? What do they measure?
Tall, cylindrical vessels with graduations scribed along the side of the cylinder; measure volume
What is more accurate: a tall cylinder with a small diameter or a short cylinder with a large diameter?
A tall cylinder with a small diameter
What principle are thermometers based on?
Liquids expand when heated
What liquid is inside common thermometers?
Mercury or colored alcohol
How are thermometer calibrated?
Measure 2 temps - BP of water and FP of water then divide the distance between those two temperatures into respective numbers (so between 0 and 100*C there would be 100 divisions)
What are pipets?
Glass vessels that are constructued and calibrated so as to deliver a precisely know volume of liquid at a given temperature
To what place is mass recorded from an analytical balance? From a top-loading balance?
0.0001, 0.001
Is the meniscus in a graduated cylinder convex or concave?
Concave
How do you correct temp of BP of water?
(760 mmHg - atmospheric pressure) (0.037C/mm)
What is the true BP after temperature correction?
100 - (BP correction) = ___ *C
Net weight =
gross - tare
What is the precision of a measurement?
A statement about the internal agreement among repeated results; a measure of the reproductibility of a given set of results
What is the simplest measure of precision?
The average deviation from the mean
How is the average deviation calculated?
sum of |value - mean| / number of values
1 mL = ____ cm^3
1
An object weighs exactly 5 g on an analytical balance that has an accuracy of 0.1 mg. To how many significant figures should this mass be recorded?
5.0001 - 5 SF
What is chromatography?
A technique by which the components of a mixture, dissolved in a fluid solvent, are separated through differences in adsorption to a solid surface (components of a mixture are separated)
Chromatography: as the fluid is passed through the solid...
The stronger the interaction of the components with the solid surface, the slower the components pass by or through the solid compared to the solvent
How is the water wicked through the paper in paper chromatography?
Capillary action
The stronger the components of the mixture interact with the cellulose of the paper, the ____ distance the components will travel along the paper.
Smaller
Retention Factor
Rf = distance traveled by spot / distance traveled by solvent front
What "water" is used for paper chromatography?
0.1 NaCl
Common Dyes
Blue #1, 2
Yellow #5, 6
Red #3, 40
Green #3
Why would the Rf values vary?
Temperature and other variables
Chromatography - General Procedure
1. Use a medicine dropper to place a drop of water on top of each piece of candy. Wait 10 minutes for water to dissolve dyes.
2. Put NaCl in a beaker (just enough to cover bottom).
3. Label chromatography paper.
4. After transferring dye to chromatography paper and letting it dry, put it in the beaker with line of dyes facing the NaCl
5. Don't let filter paper touch sides. Cover with saran wrap. Wait until NaCl travels to 1 cm from the top.
6. Remove paper. Record distances traveled by solvent and by dyes.
Why is it important to use a pencil and not a pen when doing the paper chromatography experiment?
The ink from a pen may travel up the filter paper in a similar fashion as the dye.
What do you think might happen during the solvent migration up the paper if the two long edges of the filter paper overlapped when made into a cylinder?
The molecules would interfere and the paper would also interact with the glass.
What is the purpose of the Percent Water in a Hydrated Salt Experiment?
To determine the percent by mass of water in a hydrated salt
To establish the formula of a hydrated salt
What does it mean when a salt is hydrated?
A number of water molecules are chemically bound to the ions of the salt in its crystalline structure
Water molecules that are chemically bound to the ions of the salt are called...
Waters of Crystallization
Is the number of moles of water per mole of salt constant or variable?
Constant
Formula for hydrated iron (III) chloride
FeCl3 * 6H2O
Formula for hydrated copper (II) sulfate
CuSO4 * 5H2O
What is an anhydrous salt?
A salt without water; the water molecules are so loosely bound to the ions that heat removes them to form this
Efflorescent
Hydrated salts that spontaneously (without heat) lose water molecules to the atmosphere
Deliquescent
Hydrated salts that readily absorb water
What is the formula for epsom salt?
MgSO4 * 7H2O
Can anhydrous FeCl3 form? Why or why not?
No, because the water molecules are so strongly bound to the salt that it cannot form regardless of the intensity of the heat
Percent by mass of water in the salt:
grams of water (# moles H2O 18.01g/mol) / grams of hydrated salt 100%
Gravimetric Analysis
An analytical method that relies almost exclusively on mass measurements for the analysis
What method does the Percent Water in a Hydrated Salt experiment use? How?
Gravimetric analysis; the mass of a hydrated salt is measured, the sample is heated to drive off the waters of crystallization, and the mass of remaining sample is measured again. Cycles of heating and measuring of the sample's mass are continued until reproducibility of the mass measurements is attained.
What materials are needed for the Percent Water in Hydrated Salt experiment?
Crucible and lid, tongs, bunsen burner, utility clamp, clay triangle, calculator, desiccator, balance
What is the general procedure of the Percent Water in a Hydrated Salt experiment?
Prepare a clean crucible by heating on a clay triangle
Determine the mass of the sample
Put the sample in the crucible and heat slowly and gradually
Cool in a desiccator
Measure the sample (crucible and lid first then with sample - subtract)
Reheat. Cool in a desiccator.
Re-measure the mass. If it's within 0.010 grams, you're good
Why can CaCl2, a deliquescent salt, be used as a desiccant in laboratory desiccators?
Deliquescent salts readily absorb water. Desiccators are used to remove any excess water before mass measurements. CaCl2 is a suitable desiccant because it helps remove some of the water from the crucible and sample before mass measurements.
What is the purpose of firing the crucible in the Percent Water in a Hydrated Salt experiment?
Get rid of any condensation that is naturally from the air in the lab
How does the use of crucible tongs in the experiment maintain the integrity of the analysis?
If you use your fingers, the oils from your fingers will interfere with the amount of condensation on the crucible and in the sample (the mass of the water will be too high)
Why are mass measurements performed only at room temperature?
the scales were likely calibrated at room temperature. thermal expansion can cause them to give false readings especially if you are taking very accurate measurements.
Why is the position of the crucible lid critical to the dehydration of the salt during the heating process?
If the lid were placed directly on to the crucible, the water would not be able to escape. This would result in accurate mass measurements because the sample would still be hydrated.
Percent water in the hydrated salt =
mass of hydrated - mass of anhydrous / total mass of hydrated salt * 100
Heat of Reaction / Enthalpy Change
the energy change of a reaction that occurs at constant pressure
Enthalpy Change Symbol (written out)
delta H
If heat is evolved, the reaction is __________.
If heat is absorbed, the reaction is ________.
exothermic, endothermic
What is the heat of neutralization?
The enthalpy of neutralization, or the change in energy, when an acid and a base react to form water.
What is a calorimeter?
A thermally insulated vessel
The heat liberated in the neutralization will cause a(n) ________ in the temperature of the solution and of the calorimeter.
increase
If the calorimeter were perfect, _____ would be radiated into the lab.
NO heat
What is the heat capacity of the calorimeter?
The amount of heat (the number of joules) required to raise its temperature 1 K, which is the same at 1*C.
Change in enthalpy, delta H =
negative (-) product of the temperature change, delta T, times the heat capacity of the calorimeter and its contents

= -deltaT (heat capacity of calorimeter - heat capacity of contents)
Because delta H is negative for an exothermic reaction whereas delta T is positive, a ____ sign is required in the deltaH equation
negative
How is the heat capacity of the calorimeter determined?
By measuring the temperature change that occurs when a known amount of hot water is added to a known amount of cold water in the calorimeter. The heat lost by the water = heat gained by the cold and the calorimeter. Assume no heat is lost to lab.
Heat lost by warmer water =
(Use and define variables)
(T2 - Tf) mass specific heat
T2 = temperature of the warmer water added to it
Tf = temperature after mixing
Heat gained by colder water =
(Use and define variables)
(Tf - T1) mass specific heat
Tf = temperature after mixing
T1 = temperature of a calorimeter and the cooler water
Heat lost to the calorimeter =
(heat lost by warmer water) - (heat gained by cooler water)
Heat lost to the calorimeter =
(Condensed equation)
(Tf - T1) * heat capacity of calorimeter
Heat of reaction =
(heat gained by the solution) + (heat gained by the calorimeter)
General Procedure for Heat of Neutralization
1. Construct a calorimeter (2 coffee cups inside one another, lid, thermometer with stopper so it doesn't touch bottom)
2. Measure base and add it to the calorimeter. Place lid (without thermometer) on top.
3. Measure out acid into a dry beaker. Let stand. Measure temp.
4. Insert thermometer into calorimeter. Measure temp of base. (base and acid should be +/- 0.5*C)
5. Add acid to base. Record temp every 15 seconds for 3 minutes while stirring.
6. Construct a temp vs. time curve and determine deltaT.
Purpose of Heat of Neutralization Experiment
Calculate deltaT and the heat of neutralization per mole of water formed
Define the term heat capacity.
The amount of heat (the number of joules) required to raise its temperature 1 K, which is the same at 1*C.
Define the term specific heat.
The heat required to raise the temperature of 1 g of a given substance by 1*C.
What is the difference between heat capacity and specific heat?
Heat capacity is more general; specific heat is for 1 g
Describe how you could determine the specific heat of a metal by using the apparatus and techniques in this experiment.
Obtain and weigh a metal.
Put it in boiling water and record its temperature.
Record the temperature of room temp water in a calorimeter.
Place the metal in the calorimeter once the temperatures are about equivalent.
Plot temp vs. time on a curve to determine deltaT and use the respective equations to determine specific heat -- Q = m(Cp)(deltaT) or (heat gained by water) / (heat lost by metal)
Neutralization
reaction of an acid with a base; forms water
Neutralization Reaction
H3O+(aq) + OH-(aq) --> 2H2O (l)
Purpose of Neutralization Reaction
To determine the concentration of a prepared base solution
Define standardization.
The process of determining the concentration of a solution
General Procedure of Titration of Acids and Bases
1. Standardize the base (NaOH) solution
2. Measure the volume of standardized base that is required to exactly neutralize the acid present in the unknown.
3. Use an indicator solution to determine when an acid has exactly neutralized a base (or vice versa)
Purpose of Titration of Acids and Bases
To determine the amount of acid in an unknown
Define titration.
The technique of accurately measuring the volume of a solution required to react with another reagent.
Define end point.
The point (immediately after the equivalence point) where the color changes, signifying that the base has completely neutralized the acid or vice versa.
When does a suitable indicator change color?
When equivalent amounts of acid and base are present
Example of an indicator
phenolphthalein (pink at a pH of about 9)
What is the formula for potassium hydrogen phthalate?
KHC8H4O4 (abbreviation: KHP)
How many acidic hydrogens does KHP have?
1 -- monoprotic acid
What is the molar mass of KHP?
204.2 g/mol
What is the balanced equation for the neutralization of KHP?
KHC8H4O4(aq) + NaOH(aq) --> H2O (l) + KNaC8H4O4(aq)
Define equivalence point of titration.
The point at which stoichiometrically equivalent quantities are brought together; a theoretical point determined qualitatively
Molarity units
moles solute / volume of solution liters
10^-3 mol (mmol) / 10^-3 L (mL)
% KHP =
(g KHP / mass of sample) * 100
What is parallax and why should you avoid it?
The effect of failure to level eyes with the bottom of meniscus; avoid it because it creates inaccurate readings and results
Why is it necessary to rid the distilled water of CO2?
CO2 dissolved in water creates H2CO3 (acid)
Identify each of the following as measurements of length, area, volume, mass, density, time or temp:
ns; kg/m^3; pm; 750 km^2; 83K; mm^3
time, density, length, area, temperature, volume
How many sig figs?
5.231 * 6.1
72.3/1.5
12.21 + 0.0132
31.03 + 12
2, 2, 2 decimal places, 0 decimal places
Describe the method for finding the boiling point of water at 620 mmHg.
(760 mmHg - 620 mmHg) (0.037C/mm)
How do you find standard deviation?
sum of |value - mean| / number of values
What are the two main types of chromatography?
Thin-layer chromatography (TLC)
Column chromatography (CC)
What kind of chromatography did we use?
Thin-layer chromatography (TLC)
What are the uses of chromatography?
- To separate the components of a mixture (TLC and CC)
- To determine the purity of a compound
- To see if two compounds are identical
- To monitor the progress of a reaction
The more polar a molecule, the _____ it goes up the filter paper in chromatography
more slowly
State examples of a polar stationary phase.
Alumina, silica gel, cellulose/paper
What is the mobile phase?
solvent(s) of varying polarity
State examples of a mobile phase.
NaCl, water
Come in contact with and integrate themselves into cellulose structure.
How does chromatography work?
Sample is loaded onto polar stationary phase.
Polar compounds will adsorb onto the stationary phase to a greater extent than nonpolar compounds.
Mobile (eluting) phase helps "push" the compounds either down a column (for CC) or up a plate (for TLC)
Define elution.
Process of solvent moving up the paper
What is the main concept to consider in chromatography?
Polarity
Explain why polar compounds will be more attracted to polar stationary phase more than nonpolar compounds.
Intermolecular attractive forces - a dipole-dipole interaction

Cellulose: sugar moles with hydroxyl groups -- overall negative charge hungry for positive type molecules
cellulose = polar, hydroxyl groups = sticky
Define retention.
The ability of molecules to stick to surface.
What happens if the colors smear?
They interact with one another and the retention is messed up.
Could the other ingredients in candy coatings alter Rf values from the values of the pure dyes in the solvent?
Yes because they have their own polarities, structures and molecular components. IMFs would interfere with elution and cause inaccurate results and determined Rf values
What would happen if the two long edges of the filter paper overlapped?
Samples would be contaminated and results inaccurate.
- Molecules would interact
- Purity of compound would not be able to be determined
- Progress of reaction would change
What are casts made out of?
Plaster of Paris that reacts slowly with H2O to form long interlocking crystals of CaSO4*2H2O
Define intercolate.
To interact with two components of a system (H2O interacts with the structure of salt)
Where did intercolation originate?
DNA - Point interacted with N between DNA
What is the purpose of a desiccator?
To absorb water
Mass of water (for hydrated salt experiment) =
(mass of hydrated salt) - (mass of anhydrous salt)
What happens if the lid is not kept on the crucible while cooling?
Moisture from the atmosphere will interact with the anhydrous salt, especially if the lab is humid. This will cause the mass of water (and therefore the percent of water) to be too low.
What happens if the crucible is glowing red?
It can cause the salt to decompose and make your percent of water too high.
% water by mass (hydrated salt exp)
(mass of water / mass of hydrated salt) * 100%
What is the formula of the hydrated salt?
mole ratio = moles salt / moles water
During the cooling of the fired crucible, water vapor condensed on the crucible wall before its mass measurement. The condensation did not occur following thermal decomposition of the hydrated salt in Part B. Will the reported percent water in the hydrated salt be reported too high or too low? Explain.
Too high - mass of hydrated salt would be lower than expected and the mass measurement of the crucible and lid would be too high
Hydrated salt - higher, Anhydrous salt - lower, Water lost - higher
If the crucible is handled with oily fingers after its mass measurement, would the mass of the crucible and lid be too high or too low? Would the percent water in the hydrated salt be too high or too low?
Too low mass of crucible and lid; too high percent water
- Mass of crucible, lid, hydrated salt = too high
- Mass of hydrated salt = too high
- Water lost = higher than expected (no oil after burning)
The hydrated salt is overheated and the anhydrous salt thermally decomposes, one product being a gas. Will the reported % water be too high, too low or unaffected?
Too high
- Masses would be accurate
- Water and anhydrous salt are lost so the mass of anhydrous salt would be lower than expected
- Water lost would be too high
The fired crucible is handled with oily fingers before its mass measurement. It's burned off in Part B. How does this affect the reported percent water?
It wouldn't affect it - it was burned off.
Suppose the original sample is unknowingly contaminated with a second anhydrous salt. Will the reported water to salt mole ratio be too high, too low, or unaffected?
Too low (more anhydrous salt than water lost)
- Smaller numerator OR larger denominator.
Why is thermochemistry important?
hand warmers, solar panels, ice packs
Is the neutralization of NaOH + HCl endothermic or exothermic?
Exothermic
q(rxn) =
Q(gained by H2O) + Q(gained by calorimeter)
Q =
m(Cp)(deltaT)
Write the reaction for HCl and NaOH.
HCl + NaOH --> NaCl + H2O + heat
What is the purpose of a calorimeter?
It measures the heat of a system.
What is the largest source of error in the Heat of Neutralization experiment?
Thermometer (temperature measurements)
How should the two heats of reaction for the neutralization of NaOH and the two acids compare? Why?
More heat should be evolved from the neutralization of NaOH and HCl because HCl is a strong acid and HC2H3O2 is a weak acid. Strong acids ionize completely whereas weak acids hardly dissociate. HCl, therefore, will react more fully with NaOH than HC2H3O2 causing more heat to evolve.
The experimental procedure (for the Heat of Neutralization experiment) has you wash your thermometer and dry it after you measure the temp of the NaOH solution and before you measure the temp of the HCl solution. Why?
They would react
A 50.0 mL sample of a 1.00 M solution of CuSO4 is mixed with 50.0 mL of 2.00 M KOH in a calorimeter. The temp of both solutions was 20.2C before mixing and 26.3C after mixing. The heat capacity of the calorimeter is 12.1 J/K. Calculate deltaH. -- Write the steps to solve this.
1. Find moles of CuSO4 and KOH
2. Calculate the total moles reacted
3. Calculate deltaT.
4. Calculate the mass of solution
5. Calculate Q(solution) and Q(calorimeter)
6. Total Joules Released by Reaction = Heat Gained by Soln + Heat Gained by Calorimeter (add two values from previous step)
7. deltaH = (-)(deltaT)(heat capacity of calorimeter + heat capacity of contents)
8. Divide deltaH by moles of limiting reactant.
Units for deltaH
kJ/mol
State an example of volumetric analysis.
Determination of concentrations of an unknown species in a well defined solution reaction
Define hygroscopic.
Absorbs water
What are characteristics of a primary standard?
High purity, non-hygroscopic, doesn't decompose with temp, stable, etc.
State an example of a primary standard.
KHP
Define deprotonation.
Loss of a proton -- results in a conjugate base