Question

Calculate the number of molecules of $HF$ which are contained in gaseous form due to hydrogen bonding if the density of $HF$ is gaseous phase at $1$ atm, $300K$ is $3.248$ g/litre ?

Answer 1

If vapor state of $HF$ be $(HF{)}_n$,
$P$ $\times$ Mol. wt. $=dRT$
$1$ $\times$ Mol. wt. $=3.248$ $\times$ $0.0821$ $\times$ $300$
Mol. wt. $=79.9=80$
But, Molecular weight of $HF$ is $20$.

Hence, $20\times n=80\Rightarrow n=4$
Hence, vapor state is represented as $(HF{)}_4$.