Question

A buffer solution with a pH of 12.05 consists of Na3PO4 and Na2HPO4. The volume of...

A buffer solution with a pH of 12.05 consists of Na3PO4 and Na2HPO4. The volume of solution is 200.0 mL. The concentration of Na3PO4 is 0.370 M.

What mass of Na3PO4 is required to change the pH to 12.30?

Ka = 3.6 × 10-13

_____ g

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Answer #1

Ka = 3.6 × 10-13

pKa = -log (3.6 × 10-13)

      = 12.44

pH = pKa + log [salt / acid]

12.05 = 12.44 + log [Na3PO4 / Na2HPO4]

[Na3PO4 / Na2HPO4] = 0.4074

0.200 x 0.370 / Na2HPO4 = 0.4074

Na2HPO4 = 0.1816

moles of Na2HPO4 = 0.1816

pH = pKa + log [salt / acid]

12.30 = 12.44 + log [Na3PO4 / Na2HPO4]

[Na3PO4 / Na2HPO4] = 0.7244

Na3PO4 / 0.1816 = 0.7244

moles of Na3PO4 = 0.132

mass of Na3PO4 = 0.132 x 163.94 = 21.57 g

mass of Na3PO4 is required = 21.57 g

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