Question

Assertion :Boiling point of HF is less than water . Reason: Hydrogen bonding in water is weaker than HF.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Both Assertion and Reason are incorrect

Answer 1

The correct option is B Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

Fluorine is more electronegative than oxygen. Here we would expect stronger hydrogen bonds formed with fluorine than with oxygen. But this is the reverse boiling point in $HF$ is less than the boiling point of $H_{2}O$, this is because of two reasons:

(1) In $HF$, only $1$ fluorine atom can form a hydrogen bond with the hydrogen of the neighboring $HF$ molecule. However, in water i.e. $H_{2}O$ there are two hydrogen’s which involve in hydrogen bonding with the oxygen of neighboring two water molecules and the oxygen of water can involve in hydrogen bonding with two hydrogens of neighboring two water molecules. So more hydrogen bonding per molecule of $H_{2}O$ than per molecule of $HF$, causes $HF$, to have a lower boiling point than water molecule.

Although Hydrogen bonding in water is weaker than $HF$, but the boiling point of water is more than that of $HF$

Hence option B is correct