Which of the following concepts best explains why $o$-nitrophenol is more volatile than $p$-nitrophenol?

Observe the following substances: Ethanol,acetic acid,ethylamine,trimethylamine,salicylic acid,ethanal. In the above list,the number of substances with $H$-bonding is:

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Dipole-dipole interactions are the only non-covalent interactions resulting in hydrogen bond formation.
Reason $R$: Fluorine is the most electronegative element and hydrogen bonds in $HF$ are symmetrical.

Consider the following statements:
$I.$ In gaseous phase,the vapour pressure of $CH_3COOH$ is less than expected.
$II.$ In benzene,acetic acid dimerises.
$III.$ $CH_3OH$ is more viscous than $H_2O$.
$IV.$ $CH_3OCH_3$ is non-polar but soluble in water.
Select the correct set of codes using $T$ for true and $F$ for false statements in the given sequence.

The correct order of increasing intermolecular hydrogen bond strength is .... .

$PH_3$ has a lower boiling point than $NH_3$. Why?