Question

If equal weights of oxygen and nitrogen are placed in separate containers of equal volume at the same temperature, which one of the following statements is true ?
(Molecular weight: $N_2=28$, $O_2=32$)

• A. Both flasks contain the same number of molecules.
• B. The pressure in the nitrogen flask is greater than the one in the oxygen flask.
• C. More molecules are present in the oxygen flask.
• D. Molecules in the oxygen flask are moving faster on the average than the ones in the nitrogen flask.

Answer 1

The correct option is B The pressure in the nitrogen flask is greater than the one in the oxygen flask.

For (a) and (c):
Molar mass of $N_2$ is smaller tha $O_2$ so the mass of per molecule of $N_2$ is smaller than $O_2$. Hence to make equal weight of $N_2$ and $O_2$, more number of molecules of $N_2$ had to be contained in the flask.
For (b):
We know that, for ideal gas $PV=nRT=\frac{W}{M}RT$
W and M is the mass and molecular weight of gas.
Since mass, temperature and volume of oxygen and nitrogen are same, pressure is inversely proportional to molecular weight of the gases.
We know, molecular weight of oxygen is higher than nitrogen, so the pressure of oxygen is lower than nitrogen.
For (d): Since the mass of per molecule of $N_2$ is smaller, it moves faster than $O_2$.