Question

Consider the titration of 40.0 mL 2.00M hydrocyanic acid (HCN, K_a = 4.9 x 10^-10) with 4.00M potassium hydroxide.

a) calculate the volume of base needed to neutralize this acid

b)what is the pH of the acid solution before any base is added? (weak acid problem, use approximate solution)

c) what is the pH of the solution after half of the acid is neutralized? (buffer problem)

d) what is the pH of the solution after all the acid is neutralized, at the endpoint? (weak base problem of the CONJUGATE base, use approximate solution)

Answer 1

millimoles of HCN = 40 x 2 = 80

a) to neutralize the acid :

millimoles of base = millimoles of acid

4 x V = 80

V = 20 mL

volume of base needed to neutralize the acid = 20 mL

b)

Ka of acid = 4.9 x 10^-10

pKa = -log (4.9 x 10^-10 ) = 9.31

pH = 1/2 (pKa - log C)

     = 1/2 (9.31 - log 2)

pH = 4.50

c)

At half equivalence point :

pH = pKa

pH = 9.31

d)

this is equivalence point :

here salt only remains.

salt concentration = 80 / 40 + 20 = 1.33 M

pH = 7 + 1/2 (pKa + log C)

     = 7 + 1/2 (9.31 + log 1.33)

pH = 11.72