Question

Morpholine (C4H9NO, 87.12 g.mol-1) is a weak base with Kb = 3.09 * 10-6. A sample weighing 2.51 g was dissolved in enough water to make 250.00 mL of solution, and a 50.00 mL portion of this solution was then titrated with 0.150 M HCl.

(a) What volume of HCl is required to reach the equivalence point? [1 mark]

(b) Determine the pH of the solution: (i) Before any HCl is added [1 mark] (ii) After exactly 25.00 mL of HCl is added [1 mark] (iii) At the equivalence point [1 mark]

Answer 1

C4H9NO + HCl -----> C4H9NO.HCl

Molarity of morpholine in 250 ml sample = (weight/ MW) x 1000/Vol in ml = (2.51/87.12) x 1000/250 = 0.115 M

a) C4H9NO: M₁= 0.115 V₁ = 50 ml n₁ =1

    HCl: M₂= 0.150 V₂ = ? n₂ =1

   M₁V₁/n₁=M₂V₂/n₂ =====> V₂ = (M₁V₁/n₁ ) x n₂/ M₂ = (0.115 x 50/1) x 1/0.150 = 38.33 ml

   volume of HCl is required to reach the equivalence point = 38.33 ml

b)

     (i) Before any HCl is added

     PH of weak acid