Question

Which one has the lowest boiling point?

• A. $NH_3$
• B. $PH_3$
• C. $AsCI_3$
• D. $SbH_3$

Answer 1

Answer: (B)

$PH_3$

Answer 2

In the hydrides of group number five, the boiling
points changes as
$_{238.5K}^{ \, \, NH_3 \, \, \, >} \, _{185.5 K}^{ \, \, PH_3} \, _{\, \, \, \, 254.6 K}^{ < \, \, AsH_3} \, _{ \, \, \, \, 254.6 K}^{< \, \, SbH_3} \, _{ \, \, \, \, \, \, 290 K}^{< \, \, \, BiH_3}$
$\therefore \,$ Generally mass increases then boiling point increases.
The higher boiling point of $NH_3$ is due to excessive
hydrogen bonding, so $PH_3$ have lowest boiling point
among hydrides of group number 15, i.e. nitrogen
family.