A compound has a carboxyl group an aminium group and a guani

A compound has a carboxyl group, an aminium group, and a guanidinium group. The pKas of the groups are 3.4, 9.7, and 12.7, respectively. A biochemist has 100 mL of a 0.10 M solution of this compound at a pH of 9. She adds 44 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)?

Solution

The given solution has a pH of 9. pH is neagtive logarithm of H+ ions.

pH = - log [H+]

Calculating concentration H+ ions from the given pH (9.0)

[H+] = 10^-pH

   = 10^-9

H+ concentration [H+] from 0.1 M HCl = 10^-1

Total resultant [H+] = 10^-9 + 10^-1 = 1x 10^-1

Total resultant volume after mixing = 100 ml + 44 ml = 144 ml

pH of the resultant solution = -log (resultant [H+]/resultant volume)

= - log (10^-1/144)

pH of the resultant solution will be = 3.15