\(pH = - \log {K_a} + {\text{log}}\frac{{{\text{[Salt]}}}}{{{\text{[Acid]}}}}\)
\( = - {\text{log}}1.8 \times {10^{ - 5}} + {\text{log}}\frac{{0.1}}{{0.1}}\,\,\, = - {\text{log}}\,\,1.8 \times {10^{ - 5}}\,\,\,\,\,\therefore \,\,\,pH = 4.7\)