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)

Step-by-step solution

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