Gas Laws (College Board AP® Chemistry): Exam Questions

The table above gives the molecular structures and boiling points for the compounds CS₂ and COS .

In terms of the types and relative strengths of all the intermolecular forces in each compound, explain why the boiling point of CS₂ (l) is higher than that of COS (l).

A 10.0 g sample of CS₂ (l) is put in an evacuated 5.0 L rigid container. The container is sealed and heated to 325 K, at which temperature all of the CS₂ (l) has vaporized. What is the pressure in the container once all of the CS₂ (l) has vaporized?

A student is doing experiments with CO₂ (g). Originally, a sample of the gas is in a rigid container at 299 K and 0.70 atm. The student increases the temperature of the CO₂ (g) in the container to 425 K.

Describe the effect of raising the temperature on the motion of the CO₂ (g) molecules.

Calculate the pressure of the CO₂(g) in the container at 425 K.

In terms of kinetic molecular theory, briefly explain why the pressure of the CO₂ (g) in the container changes as it is heated to 425 K.

The student measures the actual pressure of the CO₂ (g) in the container at 425 K and observes that it is less than the pressure predicted by the ideal gas law. Explain this observation.

Carbon tetrachloride, CCl₄ (g), can be synthesized according to the reaction represented below.

CS₂ (g) + 3Cl₂ (g)   → CCl₄ (g) + S₂Cl₂ (g)

A chemist runs the reaction at a constant temperature of 120°C in a rigid 25.0 L container.

Chlorine gas, Cl₂ (g), is initially present in the container at a pressure of 0.40 atm.

i) How many moles of Cl₂ (g) are in the container?

ii) How many grams of carbon disulfide, CS₂ (g), are needed to react completely with the Cl₂ (g) ?

At 30°C the reaction is thermodynamically favorable, but no reaction is observed to occur. However, at 120°C, the reaction occurs at an observable rate.

i) Explain how the higher temperature affects the collisions between the reactant molecules so that the reaction occurs at an observable rate at 120°

ii) The graph below shows a distribution for the collision energies of reactant molecules at 120°.

Draw a second curve on the graph that shows the distribution for the collision energies of reactant molecules at 30°C.

S₂Cl₂ is a product of the reaction .

i) In the box below, complete the Lewis electron-dot diagram for the S₂Cl₂  molecule by drawing in all of the electron pairs.

ii) What is the approximate value of the Cl-S-S bond angle in the S₂Cl₂molecule that you drew in part (c)(i) ? (If the two Cl-S-S bond angles are not equal, include both angles.)

CCl₄(g) can also be produced by reacting CHCl₃(g) with Cl₂(g) at 400°C , as represented by the equation below.

CHCl₃(g) + Cl₂(g)   →  CCl₄(g) + HCl(g)

At the completion of the reaction a chemist successfully separates the CCl₄(g) from the HCl(g) by cooling the mixture to 70°C, at which temperature the CCl₄(g) condenses while the HCl(g) remains in the gaseous state.

i) Identify all types of intermolecular forces present in HCl(l) .

ii) What can be inferred about the relative strengths of the intermolecular forces in CCl₄(l)  and  HCl(l) ? Justify your answer in terms of the information above.

Ethene, C₂H₄ (g) (molar mass 28.1 g/mol), may be prepared by the dehydration of ethanol, C₂H₅OH (g) (molar mass 46.1 g/mol), using a solid catalyst. A setup for the lab synthesis is shown in the diagram below.

The equation and relevant data for the dehydration reaction is given below.

C₂H₅OH (g) C₂H₄ (g) + H₂O (g)

H^o₂₉₈ = 45.5 kJ/mol_rxn

S^o₂₉₈ = 126 J/(K.mol_rxn)

A student added a 0.200 g sample of C₂H₅OH (l) to a test tube using the setup shown above. The student heated the test tube gently with a Bunsen burner until all of the C₂H₅OH (l) evaporated and gas generation stopped. When the reaction stopped, the volume of collected gas was 0.0854 L at 0.822 atm and 305 K. (The vapor pressure of water at 305 K is 35.7 torr.)

Calculate the number of moles of C₂H₄ (g)

i) that are actually produced in the experiment and measured in the gas collection tube and

ii) that would be produced if the dehydration reaction went to completion.

Calculate the percent yield of C₂H₄(g) in the experiment.

Because the dehydration reaction is not observed to occur at 298 K, the student claims that the reaction has an equilibrium constant less than 1.00 at 298 K.

Does the thermodynamic data for the reaction support the student’s claim? Justify your answer, including a calculation of ΔG°₂₉₈ for the reaction. 

The Lewis electron-dot diagram for C₂H₄ is shown below in the box on the left. In the box on the right, complete the Lewis electron-dot diagram for C₂H₅OH by drawing in all of the electron pairs.

What is the approximate value of the C–O–H bond angle in the ethanol molecule?

During the dehydration experiment, C₂H₄(g) and unreacted C₂H₅OH (g) passed through the tube into the water. The C₂H₄ was quantitatively collected as a gas, but the unreacted C₂H₅OH was not.

Explain this observation in terms of the intermolecular forces between water and each of the two gases.

A student investigates gas behavior using a rigid cylinder with a movable piston of negligible mass, as shown in the diagram above. The cylinder contains 0.325 mol  of O₂ (g) and has a volume of 7.95 L  at 25°C and 1.00 atm .

Calculate the density of the O₂ (g), in  g/L, under these conditions.

Attempting to change the density of the O₂ (g), the student opens the valve on the side of the cylinder, pushes down on the piston to release some of the gas, and closes the valve again. The temperature of the gas remains constant at 25°C. Will this action change the density of the gas remaining in the cylinder? Justify your answer.

The student tries to change the density of the O₂ (g) by cooling the cylinder to − 55°C, which causes the volume of the gas to decrease. Using principles of kinetic molecular theory, explain why the volume of the O₂ (g) decreases when the temperature decreases to − 55°C.

The student further cools the cylinder to − 180°C and observes that the measured volume of the O₂ (g) is substantially smaller than the volume that is calculated using the ideal gas law. Assume all equipment is functioning properly. Explain why the measured volume of the O₂ (g) is smaller than the calculated volume. (The boiling point of O₂ (l) is − 183°C.)

Phosphine gas, PH₃​, can be produced by reacting calcium phosphide with water:

Ca₃P₂ (s) + 6H₂O (l) → 2PH₃ (g) + 3Ca(OH)₂ (aq)

A 2.50 g sample of calcium phosphide reacts with an excess of water. The reaction occurs at 0.0094 atm and 298 K.

Identify the limiting reagent and calculate the theoretical volume of phosphine gas produced, assuming ideal gas behavior. Assume the reaction goes to completion.

The phosphine gas is collected over water at 298 K. The total pressure in the collection vessel is still 0.0094 atm.

i) Given that the vapor pressure of water at 298 K is 0.000313 atm, determine the volume of dry phosphine gas collected.

ii) Explain qualitatively how non-ideal gas behavior could further affect the final yield.

Carbon dioxide, CO₂​, is collected from the combustion of methane. A sample of CO₂​ was measured at 2.47 atm and 315 K, occupying a volume of 0.1250 L.

Calculate the mass of CO_2​ formed in this reaction.

Explain why CO₂​ behaves less ideally under the following conditions:

• At very high pressures

• At very low temperatures