Question

Calculate the pH of the buffer that results from mixing 52.8 mL of a 0.245 M solution of HCHO2 and 11.8 mL of a 0.697 M solution of NaCHO2. The Ka value for HCHO2 is 1.8×10−4.

Answer 1

Concentration after mixing = mol of component / (total volume)

M(CHO2-) after mixing = M(CHO2-)*V(CHO2-)/(total volume)

M(CHO2-) after mixing = 0.697 M*11.8 mL/(11.8+52.8)mL

M(CHO2-) after mixing = 0.1273 M

Concentration after mixing = mol of component / (total volume)

M(HCHO2) after mixing = M(HCHO2)*V(HCHO2)/(total volume)

M(HCHO2) after mixing = 0.245 M*52.8 mL/(52.8+11.8)mL

M(HCHO2) after mixing = 0.2002 M

Ka = 1.8*10^-4

pKa = - log (Ka)

= - log(1.8*10^-4)

= 3.745

use:

pH = pKa + log {[conjugate base]/[acid]}

= 3.745+ log {0.1273/0.2002}

= 3.548

Answer: 3.55