Two balloons are filled, one with pure He gas and the other by air, respectively. If the pressure and temperature of these balloons are same then the number of molecules per unit volume is

Ideal gas equation can be written as

PV = nRT ---------------(i)

n = number of moles of the gas

P = pressure of the gas

V = volume of the gas

R = universal gas constant

and T = temperature of the gas

From Eq. (i), we have $\frac{\mathrm{n}}{\mathrm{V}}=\frac{\mathrm{P}}{\mathrm{RT}}$ =constant

So, at constant pressure and temperature, all gases will contain equal number of molecules per unit volume.