Question

What is the pH of a buffer that is 0.200 M chloroacetic acid and 0.100 M sodium chloroacetate at 25°C?

I know that I have to use Henderson-Hasselbach eqn for chloroacetic acid. Why not sodium chloroacetate?

CA (chloroacetic acid) + H2O -> H3O+ ...?

Answer 1

yes that is the reaction we actually take here .if sodium chloroacetate is not given then ClCH2COO- ion concentration comes from only CA. if it is given then the ClCH2COO- ion concentration comes from CA is very less compared to direct concentration given . so we need not count from CA

let us see through ice table:

1) without common ion :

ClCH2COOH + H2O -----------------> H3O+   +   ClCH2COO-

0.20                                                      0                0              ---------------> I

-x                                                        +x                +x ----------------------> C

0.20 -x                                               x                    x -------------------> E

2)   with common ion :

ClCH2COOH + H2O -----------------> H3O+   +   ClCH2COO-

0.20                                                      0                0.10 ---------------> I

-x                                                        +x                +x ----------------------> C

0.20 -x                                             x                 0.10 + x -------------------> E

here we can use now HH equation .

pH = pKa + log [ClCH2COO- / ClCH2COOH]

pH = 2.85 + log (0.10 / 0.20)

pH = 2.55