Calculate the pH of a buffer solution that contains 0.25 M benzoic acid (C 6H 5CO 2H) and 0.15M sodium benzoate (C 6H 5COONa). [K a = 6.5 × 10 –5 for benzoic acid]

3.97

Explanation:

pH of buffer solution = pKa+Log(Cb/Ca)

pH of buffer solution = -log(Ka)+log(Cb/Ca) Equation 1

Where Ca = concentration of acid, Cb = concentration of base.

Given: Ka = 6.5×10⁻⁵, Ca = 0.25 M, Cb = 0.15 M

Substitute into equation 1

pH of buffer solution = -log(6.5×10⁻⁵)+log(0.15/0.25)

pH of buffer solution = 4.19+(0.22)

pH of buffer solution = 3.97.

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