(a) Define an acid and a base according to the Brønsted-Lowry theory, and identify the conjugate acid-base pairs in the reaction NH$_3$ + H$_2$O ⇌ NH$_4^+$ + OH$^-$. (5 marks) (b) Calculate the pH of (i) 0.01 mol dm$^{-3}$ hydrochloric acid and (ii) 0.001 mol dm$^{-3}$ sodium hydroxide, at 25 °C. (6 marks) (c) A 0.1 mol dm$^{-3}$ solution of a weak monobasic acid HA has a pH of 3.0. Calculate the acid dissociation constant $K_a$ of the acid. (4 marks)