The pH of ammonium phosphate solution, if $$pK_a$$ of phosphoric acid and $$pK_b$$ of ammonium hydroxide are 5.23 and 4.75 respectively, is ______

Ammonium phosphate $$(NH_4)_3PO_4$$ is a salt formed from the weak base ammonium hydroxide ($$NH_4OH$$) and the weak acid phosphoric acid ($$H_3PO_4$$).

For a salt of a weak acid and a weak base, the pH of the aqueous solution is given by the formula:

$$pH = 7 + \frac{1}{2}(pK_a - pK_b)$$

Here, $$pK_a$$ refers to the dissociation constant of the weak acid and $$pK_b$$ refers to the dissociation constant of the weak base. We are given $$pK_a = 5.23$$ (for phosphoric acid) and $$pK_b = 4.75$$ (for ammonium hydroxide).

Note that this formula applies when both the cation and anion hydrolyse. The ammonium ion ($$NH_4^+$$) undergoes cationic hydrolysis: $$NH_4^+ + H_2O \rightleftharpoons NH_3 + H_3O^+$$. The phosphate ion ($$PO_4^{3-}$$) undergoes anionic hydrolysis: $$PO_4^{3-} + H_2O \rightleftharpoons HPO_4^{2-} + OH^-$$.

Since both ions hydrolyse, the pH depends on the relative strengths of the acid and base. Substituting the given values into the formula:

$$pH = 7 + \frac{1}{2}(5.23 - 4.75) = 7 + \frac{1}{2}(0.48) = 7 + 0.24 = 7.24$$

Rounded to the nearest integer, the pH of the ammonium phosphate solution is $$7$$.