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

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.

A reaction between hydrogen gas and oxygen gas produces water vapor according to the balanced chemical equation:

2H₂ (g) + O₂ (g) → 2H₂O (g)

A sample containing 40.0 L of hydrogen gas is reacted with 30.0 L of oxygen gas at 25.0°C and 100 kPa.

Identify the limiting reagent and determine the volume of water vapor produced assuming ideal gas behavior.

Calculate the volume of excess gas remaining after the reaction and determine the total volume of gas in the system after the reaction is complete.

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

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

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.