Question

What is the pH change of a 0.300 M solution of citric acid (pKa=4.77) if citrate is added to a concentration of 0.170 M with no change in volume?

Answer 1

PKa = 4.77

-log[Ka] = 4.77

Ka = 10^-4.77

Ka = 1.698 x 10^-5

Concentration of citric acid = 0.300M

For weak acids, [H+] = Square root of KaxC

                             [H+] =Square root of (1.698 x10^-5 x 0.300)

                             [H+] = 2.257 x 10^-3 M

                      Taking negative log on both sides

                       -log [H+] = -log (2.257 x10^-3)

                         PH = 2.65

after adding citrate ions;

PH = PKa + log [salt]/[acid]

PH = 4.77 + log [0.170]/[0.300]

PH = 4.77- 0.247

PH= 4.52.

Change in PH = 4.52 - 2.65

Change in PH = 1.87.