We know that ideal gas equation is
$\, \, \, \, \, \, \, \, \, \, \, \, \, \, pV = nRT$
$\, \, \, \, \, \, \, \, \, \, \, \, \, \, pV = \frac{w}{M}RT$
$\, \, \, \, \, \, \, \, \, \, \, \, \, \, pM = \frac{w}{V}RT\, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \Bigg[\because \frac{w}{V}=d\Bigg]$
$\, \, \, \, \, \, \, \, \, \, \, \, \, \, pM = dRT$
$\Rightarrow \, \, \, \, \, \, \, \, d=\frac{pM}{RT}$