Question

18.62 Calculate the pH of a solution obtained by mixing 35.0 mL of 0.15 M acetic acid with 25.0 mL of 0.10 M sodium acetate.

Answer 1

This mixture of acetic acid and sodium acetate generates a buffer solution -- Acidic buffer.

The pH of this solution is solved by Henderson equation.

pH = pKa + log{[salt]/[acid]}

pKa = -logKa

= 4.74 ( from reference )

ka is dissociation constant of weak acid (here acetic acid.)

Toatl volume of the solution after mixing = 60 ml

    [salt] = 25 ml x 0.1M/ 60 ml

[acid] = 35 ml x 0.15M/ 60 ml

so pH = pKa + log{[salt]/[acid]}

= 4.74 + log{[2.5 mMol]/[5.25mMol]} ( total volume cancels out in numerator and denominator)

= 4.74 +log2.5 - log5.25

= 4.74 + 0.3979 - 0.72016

= 4.4177

pH of the resulting buffer solution  = 4.42