How many litres of water must be added to 1 litre of an aqueous solution of HCl with a pH of 1 to create an aqueous solution with pH of 2?

The pH of a 0.1 molar solution of the acid HQ is 3. The value of the ionization constant, K_a of this acid is :

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?

At 25°C, the solubility product of Mg(OH)₂ is 1.0 $$\times$$ 10^–11. At which pH, will Mg²⁺ ions start precipitating in the form of Mg(OH)₂ from a solution of 0.001 M Mg²⁺ ions?