Question

A compound has a carboxyl group, an aminium group, and a guanidinium group. The pKas of the groups are 3.6, 9.5, and 12.7, respectively. A biochemist has 100 mL of a 0.10 M solution of this compound at a pH of 8.8. She adds 34 mL of 0.1 M HCl. To the nearest hundredth of a unit, what will be the pH of the solution after addition of the HCl (assume full stirring to reach a new equilibrium)?

Answer 1

pH = pKa + log [A-]/[HA]

Carboxyl group = COO-

COO- + HCl -----------> COOH + Cl-

Given that pH = 8.8

So, pKa = 3.6

[A-] = [COO-] = 100 mL x 0.1 M = 10 mmol

[HCl] = 34 mL x 0.1 M = 3.4 mmol

A-             + HCl           -----------> HA + Cl-

10 mmol        3.4 mmol                    0

------------------------------------------------------------------

10-3.4    0                          3.4 mmol

= 6.6 mmol

Therefore,

[A-] = 6.6 mmol

[HA] = 3.4 mmol

Then,

pH = pKa + log [A-]/[HA]

     = 3.6 + log (6.6/3.4)

     = 3.88

pH = 3.88

Therefore,

pH of the solution after addition of HCl = 3.88