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Question
Calculate the pOH of 10-8 M of HCl.
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Solution
10-8 M indicates a very dilute solution. Thus, H+ ions of water cannot be ignored. But dissociation of water is suppressed due to common ion effect.
∴ [H+] ≠ 10-7 M but less than 10-7 M.
Kw = [H+] [OH–] = (10–8 + α) α
∴ α2 + α × 10-8 - 10-14 = 0
Solving above quadratic equation, we get
α = 0.95 × 10–7
∴ [H+] = 10-8 + 0.95 × 10-7 = 1.05 × 10-7 M
∴ pH = – log10[H+]
= - log10[1.05 × 10-7]
= 6.9788
∴ pOH = 14 – 6.9788 = 7.0212 ≈ 7
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