HF (hydrogen fluoride) becomes a gas at a higher temperature than does HCl (hydrogen chloride). This is inconsistent with the trend where the boiling point increases when moving down a group on the periodic table.
Explain the reason for this inconsistent behavior.

Based on the Bronsted-Lowry theory:

An acid is defined as a species which in solution tends to lose a proton (H+) while base is a species that tends to accept protons.

The given situation, HSO4⁻ acts as an acid and loses its H+ to CN⁻.

HSO4⁻ + CN⁻↔ SO4²⁻ + HCN

Here, HSO4⁻ is the acid while SO4²⁻ is its conjugate base

CN⁻ is a base and HCN is its conjugate acid