Question

Calculate the pH of the buffer that results from mixing 50.9 mL of a 0.258 M solution of HCHO2 and 15.1 mL of a 0.732 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(HCO2-) after mixing = M(HCO2-*)V(HCO2-)/(total volume)

M(HCO2-) after mixing = 0.732 M*15.1 mL/(15.1+50.9)mL

M(HCO2-) after mixing = 0.1675 M

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

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

M(HCHO2) after mixing = 0.258 M*50.9 mL/(50.9+15.1)mL

M(HCHO2) after mixing = 0.1990 M

Ka = 1.8*10^-4

pKa = - log (Ka)

= - log(1.8*10^-4)

= 3.7447

use:

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

= 3.7447+ log {0.1675/0.1990}

= 3.67

Answer: 3.67