Question

In: Chemistry

A buffer solution with a pH of 12.19 consists of Na3PO4 and Na2HPO4. The volume of...

A buffer solution with a pH of 12.19 consists of Na3PO4 and Na2HPO4. The volume of solution is 200.0 mL. The concentration of Na3PO4 is 0.320 M.

What mass of Na3PO4 is required to change the pH to 12.44?

Ka = 3.6 × 10-13

Hint: The answer is NOT 18.7 I already tried that.

Solutions

Expert Solution

Ka = 3.6 x 10-13

pKa = -log Ka = - log (3.6 x 10-13) = 12.44

Now, using henderson-hesselbalach equation

pH = pKa + log {[salt] / [acid]}

12.19 = 12.44 + log {[Na3PO4] / [Na2HPO4]}

- 0.25 = log {[0.320] / [Na2HPO4]}

0.320 / [Na2HPO4] = 10-0.25 = 0.56

[Na2HPO4] = 0.320 / 0.560 = 0.57 M

So, moles of Na2HPO4 = 0.57 M x 0.2 L =0.114 moles

Molar mass of Na2HPO4 = 142 g/mol

1 mole of Na2HPO4 = 142 g

0.114 moles of Na2HPO4 = 0.114 x 142 g = 16.19 g


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