## Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

(a) Since the gas is in a sealed container, no gas from the container can escape, and no gas from the outside can enter the container. Therefore, the number of moles of gas in the container does not change. (b) The number density of the gas in the container can be expressed as $\frac{n}{V}$. Since the number of moles of gas does not change, and the volume of the container does not change, then the number density of the gas in the container does not change.