A solution contains 10. mmol of H₃PO₄ and 5.0 mmol of NaH₂PO₄. How many milliliters of 0.10 M NaOH must be added to reach the second equivalence point of the titration of the H₃PO₄ with NaOH?

Question

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To reach the second equivalence point, all H₃PO₄ must be converted to Na₂HPO₄. This requires 2 moles of NaOH per mole of H₃PO₄. Thus, 20 mmol of NaOH is needed. With 0.10 M NaOH, 250 mL is required (20 mmol / 0.10 M = 0.20 L = 250 mL).

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