Question

Why is a monosodium phosphate solution acidic?

Answer 1

Mono sodium fosfate is one of three sodium salts that can be formed from fosforic acid:

As you will see, the three hydrogen ions (`H^+` or proton) can be 'donated' to a water molecule to make the liquid an acid one. Each step is a bit harder than the one before, because the resulting ion gets more and more negative = clinging more to the resulting `H`'s

`NaHPO_4` looks like this:

The `H_2PO_4^-`-ion can still donate `H^+`, so it will be acidic.

(pictures from Wikipedia)

Answer 2

An aqueous solution of NaH₂PO₄ is acidic because the H₂PO₄⁻ ion is a stronger acid than a base. The Na⁺ ion acts only as a spectator ion to balance the charge.

When you dissolve NaH₂PO₄ in water, it dissociates into ions.

NaH₂PO₄(s) → Na⁺(aq) + H₂PO₄⁻(aq)

The H₂PO₄⁻ ion is amphoteric. It can act as either an acid or a base.

The equation for its reaction as an acid is

H₂PO₄⁻ + H₂O ⇌ HPO₄²⁻ + H₃O⁺; `"p"K_"a" = 7.21`

The equation for its reaction as a base is

H₂PO₄⁻ + H₂O ⇌ H₃PO₄ + OH⁻; `"p"K_"b" = 11.88`

Since `"p"K_"a" < "p"K_"b"`, H₂PO₄⁻ is a stronger acid than it is a base.

This means that solutions of NaH₂PO₄ will be acidic.