Question

Which is the least volatile?
A.${\text{HF}}$
B.${\text{HCl}}$
C.${\text{HBr}}$
D.${\text{HI}}$

Answer 1

The greater is the molecular mass of a molecule the higher will be the boiling point. If the hydrogen bonding is present in any of the molecules that it will have the maximum boiling point.

Complete step by step answer:
Volatility is the measure of the tendency of a compound to form vapour. The volatility can be measured using the boiling point of the compounds. The higher the boiling point the lesser will be the volatility or vapour forming tendency of that liquid.
The boiling point depends upon the molecular mass of a compound. The higher the molecular mass the more will be the boiling point of substance. The molecular mass increases down the group hence boiling point will increase down the group in halide, but here is an exception Due to the presence of hydrogen bonding in ${\text{HF}}$ because of small size of fluorine and very high electronegativity of fluorine. While there is no hydrogen bonding present in the hydrogen chloride, hydrogen bromide and hydrogen iodide. Due to this ${\text{HF}}$ will have the maximum boiling point and will be the least volatile among all. The least boiling point will be of hydrogen chloride.

Hence, the correct option is A.

Note:
Boiling point is the point when liquid and vapour phase exist in equilibrium with each other. It is a constant point at constant atmospheric pressure. Hydrogen bonding occurs when hydrogen is attached to a very high electronegative element such as fluorine, nitrogen and oxygen. The element with which hydrogen is attached must be small in size.