Question

Calculate the pH at the equivalence point in the titration of the weak base, B, having a K_b of 1.00 x 10⁻⁸ when the initial formal concentration of the B titrand is 0.01000F and its initial volume is 0.1000 L. The titrant is standardized HCl with a concentration of 0.1000 M.

Answer 1

Reaction between the acid and base will go as follows

B + HCl BH⁺Cl^-

At the equivalnec point all B will be concumed and will form only BHCl

Since the concentration of the base is 0.01 F which is similar to 0.01 M

and the volume of 0.1L

we will need

0.1 x 0.01 M = 0.1 x V2 HCl at the equivalence point

V2 = 0.01 L

So now the total volume of the liquid is

0.1L + 0.01 L = 0.11L

BHCl will be 0.01 x 0.1L = 0.001 M

Now the BHCl will ionise in solution to give and the ICE table will give us the concentrations

BH⁺ + H₂O B + H₃O⁺

initial 0.001 0 0

Change -x +x +x

equilibrium 0.001 -x x x

Ka for ammonium chloride is 5.6 x 10^-10

Ka = x²/0.001-x

5.6 x 10^-10 = x²/0.001-x ( x in the denominator is too small and can be ignored

x² = 5.6 x 10^-10 x 0.001

x = 5.6 x 10^-13

x = 7.48 x 10^-7 This is the concentration of H₃O⁺ from this we can calculate pH as

pH = -log [H₃O⁺]

pH = -log (7.48 x 10^-7)

pH = 6.12