Key concept: identifying a $\mathrm{P-H}$ bond in oxyacids of phosphorus

In oxyacids of phosphorus, hydrogen can be attached either:

• to oxygen as $\mathrm{O-H}$, or
• directly to phosphorus as $\mathrm{P-H}$.

Only hydrogens directly bonded to phosphorus count as $\mathrm{P-H}$ bonds.

Useful structural forms

Let us write the common structures of the given acids.

(A) $\mathrm{H_3PO_4}$

Structure: $\mathrm{O=P(OH)_3}$

Here, all three hydrogens are in $\mathrm{OH}$ groups. So, there is no $\mathrm{P-H}$ bond.

Therefore, A is incorrect.

(B) $\mathrm{H_3PO_3}$

Structure: $\mathrm{HPO(OH)_2}$

Here:

• two hydrogens are in $\mathrm{OH}$ groups,
• one hydrogen is directly attached to phosphorus.

So, it contains one $\mathrm{P-H}$ bond.

Therefore, B is correct.

(C) $\mathrm{H_4P_2O_7}$

This is pyrophosphoric acid. Structure can be written as: $\mathrm{(HO)_2P(=O)-O-P(=O)(OH)_2}$

All hydrogens are attached to oxygen as $\mathrm{OH}$. There is no $\mathrm{P-H}$ bond.

Therefore, C is incorrect.

(D) $\mathrm{H_3PO_2}$

Structure: $\mathrm{H_2PO(OH)}$

Here:

• one hydrogen is in an $\mathrm{OH}$ group,
• two hydrogens are directly attached to phosphorus.

So, it contains $\mathrm{P-H}$ bond(s).

Therefore, D is correct.

Final selection

Compounds containing $\mathrm{P-H}$ bond(s): $\boxed{\mathrm{H_3PO_3} \text{ and } \mathrm{H_3PO_2}}$

So the correct options are: $\boxed{B, D}$

Comparison with stored correct answer

Stored correct answer: B, D

My derived answer: B, D

Hence, they agree.