Question

Calculate the pH and the equilibrium concentrations of H2C6H5O7-, HC6H5O72- and C6H5O73- in a 0.2370 M aqueous citric acid solution.

For H3C6H5O7, Ka1 = 7.4×10-3, Ka2 = 1.7×10-5, and Ka3 = 4.0×10-7

pH =

[H2C6H5O7-] = ____M

[HC6H5O72-] = ____M

[C6H5O73-] =_____ M

Answer 1

H3C6H5O7 ---------------------------------> H2C6H5O7-   + H+

0.2370                                                     0                         0 ---------------> initial

0.2370 -x                                                  x                          x -------------------> equilibrium

Ka1 = x^2 / 0.2370-x = 7.4×10-3

x^2 + 7.4×10-3 x - 1.75 x 10^-3 = 0

x = 0.0383

[H+] = 0.0383 M

pH = -log [H+]

pH = 1.42

[H2C6H5O7-] = 0.0383 M

[HC6H5O72-] = 1.7×10^-5

[C6H5O73-] = 1.78 x 10^-10 M