(b)
Given $p_{s}$ =19.8 mm
$n_{A} = 0.1$
$n_{B} = \frac{178.2}{18} = 9.9\$
According to Raoult’s law
$\frac{p_{s} - p}{p_{s}} = \frac{n_{A}}{n_{A} + n_{B}}$
$\frac{19.8 - p}{19.8} = \frac{0.1}{9.9 + 0.1}$
or 198-10 p = 19.8 $\times$0.1
10 p = 198-1.98
10 p =196.02
p = 19.602 mm