Three reactions involving $$H_2PO_4^−$$ are given below :

(i) H₃PO₄ + H₂O $$\to$$ H₃O⁺ + $$H_2PO_4^−$$

(ii) $$H_2PO_4^−$$ + H₂O $$\to$$ $$HPO_4^{2−}$$ + H₃O⁺

(iii) $$H_2PO_4^−$$ + OH^- $$\to$$H₃PO₄ + O^2-

In which of the above does $$H_2PO_4^−$$ act as an acid?

Solid Ba(NO₃)₂ is gradually dissolved in a 1.0 $$\times$$ 10^-4 M Na₂CO₃ solution. At what concentration of Ba²⁺ will a precipitate begin to form ?
(K_sp for BaCO₃ = 5.1 $$\times$$ 10⁻⁹ )

Four species are listed below
i. $$HCO_3^−$$
ii. $$H_3O^+$$
iii. $$HSO_4^−$$
iv. $$HSO_3F$$
Which one of the following is the correct sequence of their acid strength?

The pK_a of a weak acid, HA, is 4.80. The pK_b of a weak base, BOH, is 4.78. The pH of an aqueous solution of the corresponding salt, BA, will be