Question

Which of the following oxy-salt does not exist :

• A. Sodium dihydrogen phosphate
• B. Sodium dihydrogen hypophosphite
• C. Potassium hydrogen sulphite
• D. Sodium metaborate

Answer 1

Answer: (B)

Sodium dihydrogen hypophosphite

Answer 2

The existence of an oxy-salt depends on the basicity of its parent oxy-acid.

1. Sodium dihydrogen phosphate: Parent acid is phosphoric acid ($\text{H}_3\text{PO}_4$), which is tribasic. It can form $\text{H}_2\text{PO}_4^{-}$ (dihydrogen phosphate) by losing one $\text{H}^+$. So, $\text{NaH}_2\text{PO}_4$ exists.

2. Sodium dihydrogen hypophosphite: Parent acid is hypophosphorous acid ($\text{H}_3\text{PO}_2$). Its structure ($\text{H}_2\text{P(O)OH}$) reveals it has only one acidic hydrogen (attached to oxygen). Thus, it is monobasic. It can only form the hypophosphite ion ($\text{H}_2\text{PO}_2^{-}$) by losing one $\text{H}^+$. The term "dihydrogen hypophosphite" implies the presence of two acidic hydrogens in the anion, which is impossible for a monobasic acid. Therefore, this salt does not exist.

3. Potassium hydrogen sulphite: Parent acid is sulphurous acid ($\text{H}_2\text{SO}_3$), which is dibasic. It can form $\text{HSO}_3^{-}$ (hydrogen sulphite) by losing one $\text{H}^+$. So, $\text{KHSO}_3$ exists.

4. Sodium metaborate: Parent acid is metaboric acid ($\text{HBO}_2$), which is monobasic. It forms $\text{BO}_2^{-}$ (metaborate) by losing one $\text{H}^+$. So, $\text{NaBO}_2$ exists.

Hence, Sodium dihydrogen hypophosphite does not exist.