Question

The number of hydrogen bonds formed by a molecule each of H${}_2$O, HF and NH${}_3$ respectively are :

• A. $2,2,3$
• B. $4,2,3$
• C. $2,1,3$
• D. $4,4,3$

Answer 1

The correct option is B $4,2,3$

In Water oxygen atom has two lone pairs and two hydrogen atoms, meaning that the total number of hydrogen bonds of a water molecule is up to four.

In HF, each molecule has one δ+ hydrogen and three active lone pairs. In the liquid as a whole, there are therefore three times as many lone pairs are there are δ+ hydrogens. On average, then, each molecule can only form one hydrogen bond using its δ+ hydrogen and one involving one of its lone pairs. The other lone pairs are essentially wasted. Each ammonia molecule can form one hydrogen bond using its lone pair and one involving one of its δ+ hydrogens. The other hydrogens are wasted.