At 25°C, 20.0 mL of 0.2 M weak monoprotic acid HX is titrated against 0.2 M NaOH. The pH of the solution (a) at the start of the titration (when NaOH has not been added) and (b) when 10 mL of NaOH is added respectively, are: Given: K_a = 5 10^ -4 , pK_a = 3.3, (a) (b)
B. 2.0 3.3
For part (a), at the start of the titration, the solution contains only the weak acid HX. The concentration of HX is C = 0.2 M. Using the formula for the hydrogen ion concentration of a weak acid with 1: [H^+] = K_a C = 5 10^ -4 0.2 = 10^ -4 = 10^ -2 M pH = - (10^ -2 ) = 2.0 For part (b), when 10 mL of 0.2 M NaOH is added to 20 mL of 0.2 M HX: Initial millimoles of HX = 20 0.2 = 4 mmol Millimoles of NaOH added = 10 0.2 = 2 mmol The added NaOH neutralizes half of the HX to form NaX, creating an acidic buffer. Millimoles of HX remaining = 4 - 2 = 2 mmol Millimoles of NaX formed = 2 mmol Using the Henderson-Hasselbalch equation: pH = pK_a + ( [ Salt ] [ Acid ] ) Since the millimoles of salt and acid are equal, (1) = 0. pH = pK_a = 3.3 Answer: 2.0 3.3
Related: Chemistry — Ionic Equilibrium · All PYQ Banks