# Problem: What is the pH of a 0.40 M solution of benzoic acid, C6H5COOH? (The Ka value for benzoic acid is 6.3 x 10-5.)

###### FREE Expert Solution

Step 1. Calculate [H+]:

C6H5COOH   ⇌   C6H5COO-   +  H+

$\overline{){{\mathbf{K}}}_{{\mathbf{a}}}{\mathbf{=}}\frac{\mathbf{products}}{\mathbf{reactants}}}\phantom{\rule{0ex}{0ex}}{\mathbf{K}}_{{\mathbf{a}}}\mathbf{=}\frac{\left[{H}^{+}\right]\left[{C}_{6}{H}_{5}{\mathrm{COO}}^{-}\right]}{\left[{C}_{6}{H}_{5}\mathrm{COOH}\right]}\mathbf{=}\mathbf{6}\mathbf{.}\mathbf{3}\mathbf{×}{\mathbf{10}}^{\mathbf{-}\mathbf{5}}\phantom{\rule{0ex}{0ex}}\frac{\mathbf{\left[}\mathbf{x}\mathbf{\right]}\mathbf{\left[}\mathbf{x}\mathbf{\right]}}{\left[0.40\right]}\mathbf{=}\mathbf{6}\mathbf{.}\mathbf{3}\mathbf{×}{\mathbf{10}}^{\mathbf{-}\mathbf{5}}\phantom{\rule{0ex}{0ex}}\sqrt{{\mathbf{x}}^{\mathbf{2}}}\mathbf{=}\sqrt{\left(6.3×{10}^{-5}\right)\left(0.40\right)}$

x = 5.020x10-3 M = [H+]

83% (423 ratings)
###### Problem Details

What is the pH of a 0.40 M solution of benzoic acid, C6H5COOH? (The Ka value for benzoic acid is 6.3 x 10-5.)