Question

Which buffer solution has maximum pH?
a) mixture which is 0.1 MCH_​3COOH and 0.1 M CH₃COONa [ pK_a (CH₃COOH) = 4.74]
b) mixture which is 0.2 M in CH₃COOH and 0.2 M in CH₃COONa
c) mixture which is 0.1 M in NH₄Cl and 0.1 M in NH₄OH [pK_b (NH₄⁺) = 9.26]
d) all the solution have equal Ph which is 4.74

Answer 1

The pH of buffer solutions can be measured by Henderson-Hasselbalch equation:
$\mathrm{pH}={\mathrm{pK}}_{\mathrm{a}}+\log \left(\frac{\left[\mathrm{base}\right]}{\left[\mathrm{acid}\right]}\right)$

$$\begin{gathered} \mathrm{pH}\mathrm{of}\mathrm{mixture}1=4.74+\log \left(\frac{[.1\mathrm{M}]}{[.1\mathrm{M}]}\right)=4.74 \\ \mathrm{pH}\mathrm{of}\mathrm{mixture}2=4.74+\log \left(\frac{[.2\mathrm{M}]}{[.2\mathrm{M}]}\right)=4.74 \\ {\mathrm{pK}}_{\mathrm{a}}+{\mathrm{pK}}_{\mathrm{b}}=14 \\ \mathrm{hence}{\mathrm{pK}}_{\mathrm{a}}\mathrm{for}\mathrm{mixture}3=14-9.26=4.74 \\ \mathrm{pH}\mathrm{of}\mathrm{mixture}3=4.74 \end{gathered}$$

Hence the answer is (d).