Question

At 300 K and 1.00 atm, the density of gaseous HF is 3.17 g/L. Planar HF molecules are associated with H-bonding. Overall molecular weight of the associated molecule is determined. Hence, number of HF molecules associated will be _______.

Answer 1

$PV=nRT=\frac{w}{M}RT$

$\therefore p=\frac{w}{VM}RT=\frac{dRT}{M}$

$\therefore$ M (molecular weight) $=\frac{dRT}{P}=\frac{3.17\times 0.0821\times 300}{1}$
$=78.08gmo{l}^{-1}$

Number of HF molecules associated via H-bonding, $n=\frac{78}{20}\approx 4$

$\Rightarrow n=4$.