Kinetic Molecular Theory Question 3

The reaction between $\ce{NO(g)}$ and $\ce{O2(g)}$ to produce $\ce{NO2(g)}$ in a rigid vessel is represented in the diagram above. The pressure inside the container is recorded using a pressure gauge. Which of the following statements correctly predicts the change in pressure as the reaction goes to completion at constant temperature, and provides the correct explanation?

The pressure will increase because the product molecules have a greater mass than either of the reactant molecules.

The pressure will decrease because there are fewer molecules of product than of reactants.

The pressure will decrease because the product molecules have a lower average speed than the reactant molecules.

The pressure will not change because the total mass of the product molecules is the same as the total mass of the reactant molecules.

Approach

The pressure of a container is based solely on the number of gas molecules if volume and temperature are held constant.

PV=nRT

\frac{P}{n} = \frac{RT}{V}

\frac{P}{n} = k

P \propto n \text{ if } V, R, \text{and } T \text{ are constant}

The pressure of the container decreases since there are less molecules exerting pressure on the walls of the container.

Since the temperature is constant, the average speed of the molecules will stay the same.