Question

Calculate the molecular weight of $HF$ if density of $HF$ gas is $3.17g/L$ at $300K$ and $1.0atm$ . Comment on the result.

Answer 1

$\because$ $PV=\frac{W}{m}RT$

or $P=\frac{W}{V.M}RT$

or $M=\frac{dRT}{P}=\frac{3.17\times 0.0821\times 300}{1}=78.08$

Molecular weight of $HF$ in gaseous state is $78.08$ , whereas its normal molecular weight is $1+18=19$

Thus , $HF$ in gaseous state from a tetramer due to strong $H$ - bonding .