A 10.-liter flask at a given temperature and pressure contains 6.0 × 1023 molecules of hydrogen gas. Under the same conditions of temperature and pressure, how many molecules would a 10.-liter flask of nitrogen gas contain?

The same number of molecules, 6.0 × 10²³ molecules.

Explanation:

The amount of any given gas that can be stored in a container depends on the temperature, pressure and volume of the container. It does not depend on the nature (or identity) of the gas.

So if a 10-liter flask contains 6.0 × 10²³ molecules of hydrogen gas, it will contain the same amount of molecules of any other gas when temperature and pressure remain constant.