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Q.
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
$PV = nRT\,\,\, ...(i)$
In this equation,
$n =$ 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
$\frac{ n }{ V }=\frac{ P }{ RT }=$ constant
So, at constant pressure and temperature, all gases will contain equal number of molecules per unit volume.
This result is nothing but Avogadro's law.