Why does the mixture (pH 6.23) come out acidic, near pH 6?

The acidic sample has far more H+ so it dominates

Larger volume always sets the pH

2076 Chaitra · 4 marks

pH of mixture: 700 ml pH 6 and 500 ml pH 8

Short answer

Combine hydrogen-ion amounts: final H+ = (700e-6 + 500e-8)/1200 = 5.875e-7 mol/l. Taking the negative log gives pH = 6.23.

A water sample of 700 ml with pH 6 is mixed to another water sample of 500 ml with pH 8. What will be the pH of the mixture?

pH is $-\log_{10}[H^+]$ . When two samples are mixed, the hydrogen-ion amounts (moles) add, so the final $[H^+]$ is the volume-weighted average. (Buffering by alkalinity is ignored, as is usual in this type of problem.)

Sample 1: 700 ml, pH 6, so $[H^+] = 10^{-6}$ mol/l. Sample 2: 500 ml, pH 8, so $[H^+] = 10^{-8}$ mol/l.

\begin{aligned} [H^+]_{mix} &= \frac{700\times10^{-6} + 500\times10^{-8}}{700+500} \\ &= \frac{7.0\times10^{-4} + 5.0\times10^{-6}}{1200} = 5.875\times10^{-7}\ \text{mol/l} \end{aligned}

pH = -\log_{10}(5.875\times10^{-7}) = 6.23

Answer: pH of the mixture = 6.23 (acidic, because the acidic sample dominates).

✓ Answer checked by Claude Opus 5.5 (high effort)

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