CHEM 102L Final Exam Review

Experiment 0: Safety in the Lab and General Error

When should you not wear gloves in the laboratory setting? (select all that apply)

a. When weighing out reagents at the balances

b. When entering data into your ELN on your laptop

c. When adding a Kimwipe to the solid waste container

d. When handling clean glassware

e. When opening the door to go to the restroom

f. When writing notes on scratch paper using a pen and paper

g. When adding chemicals to the liquid waste

b, e, f

You are adding 0.1 M NaOH to a cuvette containing crystal violet. While using the micropipette, you spill 100µL onto your bare arm. What should you do?

wash off w/ water, tell TA

Iron Man is taking an evening laboratory session and wants to bring a snack to help him make to through the evening. When should he be allowed to have food at the bench top in the lab?

a. If the experiment work is over and cleaned up

b. At the beginning because it is dinner time and crime fighting is exhausting

c. Only if the food remains in the students backpack while at the bench top

d. Never

e. Iron Man is hypoglycemic and should be allowed to eat at the bench to prevent health issues

d

______ error arises from a flaw in the equipment or in the experimental design. This type of error ____ be detected and corrected. Provide several examples.

systematic, can

sig figs, old equipment

______ error arises from uncontrolled, and often times uncontrollable, variables in an experiment which has equal chance of being positive and negative. This type of error ____ be detected and corrected. Provide several examples.

random, can't

inaccurate reading of buret (miniscus), equipment noise (movement)

Define accuracy

how close a measurement is to the true value

Define precision

how close the measured values are to each other

You determined the molar mass of unknown fatty acid #3 dissolved into stearic acid to be 219.4 g/mol. Unknown fatty acid #3 was myristic acid which has a molar mass of 228.37 g/mol. What was your percent error to the correct number of significant figures?

a. 3.9%

b. 3.93%

c. 4.1%

d. 4.08%

b

Experiment 11: Gas Laws

You sealed an Erlenmeyer flask that was determined to have a volume of 272.2 mL with a stopper. The density of air at 25°C is 0.001185 g/mL. Air has an effective molar mass of 28.96 g/mol. What amount of air was sealed in the flask?

0.01114 mol

Using the appropriate graph above, what is the ideal gas constant from this experiment?

.081 Latm/molK

What is the name of the law that relates temperature and pressure?

a. Charles' Law

b. Gay-Lussac's Law

c. Avogadro's Law

d. Boyle's Law

e. Motley's Law

b

Which of the following represents possible sources of error during this experiment? (select all that apply)

a. Air was used as a model of an ideal gas

b. Air leaks in the pressure/temperature apparatus

c. The incorrect size of Erlenmeyer flask was used during the experiment

d. The water was not completely boiling when heating the flask

e. Excess water vapor was sealed in the flask prior to heating

a, b

What is the definition of absolute zero? What does this imply about the pressure at absolute temperature?

theoretical temp where there is no motion & no heat; no pressure

Experiment 12: Intermolecular Forces

Vaporization is a(n) _______ process.

endothermic

True or False. As molecules escape from the liquid to the vapor phase, the average kinetic energy of the solution is lower and the temperature of the solution is higher than the original solution.

false

If molecule A has an enthalpy of vaporization, ∆Hvap, that is larger than molecule B, what does this imply about the intermolecular forces of molecule A relative to molecule B?

a. Higher

b. Lower

c. Equal

d. Not enough information to determine

a

Based on the graph below, what is the ∆Hvap of the liquid being studied?

a. 40.8 kJ/mol

b. -40.8 kJ/mol

c. 59,800 kJ/mol

d. -4910.1 kJ/mol

e. 49010 kJ/mol

a

Liquids with __________ intermolecular forces have a ________ enthalpy of vaporization, and the molecules escape into the vapor phase _________.

a. Stronger, larger, with difficulty

b. Stronger, smaller, easily

c. Weaker, larger, easily

d. Weaker, smaller, with difficulty

a

For several molecules with the same chemical formula, how does the degree of branching influence the relative intermolecular forces? Why?

IM forces based on surface area

> branching = < surface area = < IM forces

Experiment 13: Colligative Properties

Define molality. What are its units?

mol of solute/kg of solvent

Which of the following is not a colligative property?

a. Osmotic Pressure

b. Freezing Point Depression

c. Boiling Point Elevation

d. UV/Visible Absorption

e. Vapor Pressure Reduction

d

Which of the following salts would cause the largest freezing point depression in water (Kf= 1.86°C/m)?

a. 0.6 m LiCl

b. 0.5 m Mg(Cl)2

c. 0.4 m Cu(Cl)2

d. 1.2 m acetone

d

In this experiment, how did you prevent super cooling? Why is super cooling problematic?

insulation jacket; cools to quickly (crystallize w/ seed crystal)

Many antifreeze solutions we use in our automobiles are a mixture of ethylene glycol (MW= 62.07 g/mol, i=1) and water (Kf= 1.68°C/m). A brand fo antifreeze that is 28.6% ethylene glycol by mass will freeze at -12°C. What is the molality of the ethylene glycol in this solution?

6.45 mol/kg

At the same molality of ethylene glycol as above, by how much would you depress the freezing point of cyclohexanol (Kf= 39.3°C/m, Tpure= 25.93°C)?

-228°C

True or False. A colligative property is a property that describes the behavior of a solution as opposed to its individual components

true

True or False. A colligative property depends on both the identity and the amount of the solute in the solution.

false

A student was conducting Experiment 13: Colligative Properties. Into a single test tube, the student added the stearic acid, measured the freezing point of the solvent, and made three subsequent additions of an unknown fatty acid while measuring the freezing point after each addition. Below is the student's data. Unfortunately, the value of the molar mass of the unknown is significantly different in each calculation. What is a reasonable explanation for this result?

a. The experiment was carried out incorrectly. The dramatic temperature changes indicated whtat super cooling occured.

b. The amount of unknown added for each trial should have been the same. The differences in the masses of the unknown fatty acid is the cause of the differences in the calculated molar mass.

c. The calculation of the molar mass did not account for the total amount of unknown fatty acid added for additions 2 and 3. The student made a calculation error.

c

You have 8.00 g of stearic acid in a test tube. You weigh out 0.800 g of an unknown fatty acid and record the value in your ELN. While transferring the unknown fatty acid to the test tube, you accidently spill 0.100 g, but do not record this in your ELN. You measure a ∆Tf= 2.00°C. How does this error affect the calculated molar mass of the unkown? (HINT: Calculate the molar mass first based on the value you recorded. Then, calculate the molar mass accounting for the 0.100g loss. Finally, compare the values.)

Experiment 14: Chemical Kinetics

Beer's law defines a mathematical relationship between ________ and ________.

absorbance, concentration

In the reaction between crystal violet and sodium hydroxide, what property were you monitoring as a function of time?

absorbance

A 1.0x10⁻⁵ solution of crystal violet has an absorbance of 0.563. Into a new cuvette, 2,000µL of the crystal violet was added to 1,500 µL of distilled water. What is the absorbance of this new solution?

a. 0.75

b. 0.32

c. 0.24

d. 0.98

a?

In Experiment 14: Chemical Kinetics, it was necessary for you to create a calibration curve that would allow you to convert measured absorbance values to concentration. To do this, you prepared five solutions of a known concentrations and measured their absorbance. If done correctly, a plot of absorbance vs concentration for these data should provide:

a. A straight line with a negative slope, in which the slope represents that rate at which the colored crystal violet is disappearing from solution and the y-intercept represents the initial concentration

b. A straight line with a positive slope, in which the slope represents the product of the molar absorptivity coefficient and the path length of the light through the solution.

c. A polynomial curve in which the terms represent the molar absorptivity, the path length of the light through the solution, and the wavelength of light used to probe the sample

d. A straight line with a positive slope, in which the slope represents the measured rate constant for the reaction and the intercept in the inverse of the reaction

d

What is the rate law for the following reaction?

aA + bB → cC + dD

a. rate= k a[A]b[B]

b. rate= k [A]^a[B]^b[C]^c[D]^d

c. rate= k [C]^c[D]^d

d. rate= k [A]^a[B]^b

d

The rate for the bleaching reaction between crystal violet and NaOH is Rate= k[OH-]^x[CV]^y. NaOH is in excess for this experiment allowing for the order of crystal violet to be determined through a simplified rate law with a new rate constant. What is the new rate law and what is the rate constant called?

rate= K' [CV]y

Using the integrated rate laws method, you determined that the reaction was first order with respect to the crystal violet concentration. The slope of the linear regression was -0.0020 s⁻¹. For the data that you acquired, you mixed 500µL of a 1.0M NaOH solution with 3,000µL of a dilute crystal violet solution in the cuvette. what is the overall reaction rate constant?

a. 0.014 M⁻¹s⁻¹

b. 0.002 M⁻¹s⁻¹

c. -0.014 M⁻¹s⁻¹

d. -0.002 M⁻¹s⁻¹

e. 0.002 s⁻¹

d

The rate expression for a reaction was found to be rate= k[A]⁰[B]². If the concentration of A was increased by a factor of 3 and the concentration of B was increased by a factor of 4, how would you expect the rate of the reaction to change?

a. The rate is constant

b. the rate increases by a factor of 7

c. the rate increases by a factor of 12

d. the rate increases by a factor of 16

e. the rate increases by a factor of 48

d

True or False. During the reaction of crystal violet with sodium hydroxide, the solution started off completely colorless and became increasingly more violet during the couurse of the reaction.

false

Experiment 15: Chemical Equilibria

In Experiment 15: Chemical Equilibria, which physical property of which compound were you monitoring?

concentration of [Fe(NCS)]²⁺

In Experiment 15: Chemical Equilibria, the experimental purpose for creating a Beer's Law plot for [Fe(NCS)]²⁺ is best described as:

a. To demonstrate the linear relationship between the concentration of [Fe(NCS)]²⁺ and absorbance

b. To determine the concentration of [Fe(NCS)]²⁺ in a solution at equilibrium

c. To determine the initial concentration of [Fe(NCS)]²⁺ in a solution before equilibrium is achieved

d. To follow the progress fo the reaction of Fe³⁺ and NCS⁻

a

In Experiment 15: Chemical Equilibria, you used Le Chatelier's principle when preparing your solutions for the standard curve (Beer's Law plot).

What is Le Chatelier's principle?

when there's a change in the system, it will change to meet equilibrim

The equilibrium below was the reaction of interest in Experiment 15. In the preparation of your standard solutions, which reactant was used in excess? Which reactant was essentially converted entirely to product?

Fe³⁺+SCN⁻↔[Fe(NCS)]²⁺

True or False. The Keq changes as the initial concentrations of Fe³⁺ and SCN⁻ change.

true

True or False. At equilibrium, the concentrations of the reactant(s) and product(s) are constant.

true

True or False. Equilibrium is a static process and no reactions occur once equilibrium is achieved.

false

In Experiment 15: Chemical Equilibria, what were the standard and equilibrium solutions used for, respectively?

What is the definition of λmax? How was it used in this experiment?

wavelength where absorbance is max; to determine the concentration at that wavelength

When calibrating the spectrometer, you accidently blanked on a slightly colored solution that had an absorbance of 0.1 at the λmax. This means that every absorbance measured for the standard curve will be 0.1 lower than what the value should be. How will this influence your standard curve relative to a spectrometer that was calibrated correctly?

a. The slope of the standard curve will be unchanged, but the y-intercept will be -0.1

b. The slope will be smaller, but the y-intercept will be unchanged

c. The slope will be smaller and the y-intercepts will be 0.1

d. The slope and the y-intercept will be unaffected

a

Experiment 16: Acid/Base Titrations

When calculating the volume of titrant added at the end point of an acid-base titration, the volume reading on the burette is at 28.98 mL before the titration and 37.52 mL at the end point. How mnay significant figures should be recorded for the volume of titrant added?

a. 1

b. 2

c. 3

d. 4

d

You reach the equivalence point after titrating 24.21 mL of 0.09658M NaOH into a solution containing 0.3010g of an unknown, monoprotic acid dissolved in 50.00 mL of water. What is the molecular weight of the unknown acid?

128.7 g/mol

Why must the NaOH solution be standarized prior to use as the titrant to determine the identity of an unknown acid?

Explain the difference between the end point and the equivalcne point. Does the end point and equivalence point always occur at the same pH? Why or why not?

endpoint- by indicator (turn pink)

equivalence point- moles of acid equal moles of base

In this experiment, the indicator was _________ and changed from ________ in acidic solutions to _________ in basic conditions.

phenolpthalein, clear, pink

What was the primary and secondary standards in both experiment 16.1 and 16.2?

a. Primary= NaOH; Secondary= potassium hydrogen phthalate (KHP)

b. Primary= NaOH; Secondary= unknown acid

c. Primary= unknown acid; Secondary= NaOH

d. Primary= potassium hydrogen phthalate (KHP); Secondary= unknown acid

e. Primary= potassium hydrogen phthalate (KHP); Secondary= NaOH

d

Which properties are necessary qualities of a primary standard? (select all that apply)

a. Possess a high level of purity

b. Are soluble in water or common solvents at suitable concentrations

c. Are typically liquids

d. Are stable in air

e. Can be quantitatively weighed out

f. Completely react with the substance to be standardized

g. Are a solid

h. Produce an equivalence point that occurs at a pH=7

a, b, d, e, f

You prepare a solution of KHP by dissolving0.740g of potassium hydrogen phthalate (KHP, 204.221 g/mol) in 25.0 mL of distilled H₂O with phenolphthalein. It takes 17.33mL of your ~0.1M NaOH stock solution for the solution to turn a faint shade of pink. What is the concentration of [OH⁻] in your stock solution?

a. 1.07 M

b. 0.0427 M

c. 0.105 M

d. 0.209 M

d