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What volume (in litres) of 0.2 M NaOH solution must be added to 500 mL of 0.5 M NaOH so that 300 mL of the resulting solution is exactly neutralised by 20 mL of 2 M H3PO4?

1. 0.1
2. 0.2
3. 0.3
4. 0.4

Correct answer: 0.3

Solution

20 mL of 2 M H3PO4 = 0.04 mol acid needs 0.12 mol OH- in 300 mL, so final NaOH = 0.4 M; mixing 500 mL of 0.5 M with V of 0.2 M to reach 0.4 M gives V = 0.3 L.

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