Phosphoric acid is a triprotic acid with the following pK_a values: pK_a1 = 2.14

Question

Phosphoric acid is a triprotic acid with the following pK_a values: pK_a1 = 2.148 pK_a2 = 7.198 pK_a3 = 12.375 You wish to prepare 1.000 L of a 0.0500 M phosphate buffer at pH 7.400. To do this, you choose to mix the two salt forms involved in the second ionization, NaH_2PO_4 and Na_2HPO_4, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture? What other combination of phosphoric acid and/or its salts could be mixed to prepare this buffer? (Check all that apply).

Explanation / Answer

Using Henderson-Hasselbalch Equation:-
pH = pKa + log{[salt form]/[Acid form]}

Here salt form = Na2HPO4 & acid form = NaH2PO4

pKa = 7.198

Thus, 7.4 = 7.198 + log{x/y}

or, log{x/y} = 0.202

or, x/y = 100.202 = 1.6.........(1)

Now, Total moles of Na2HPO4 & NaH2PO4 = 0.05

or, x + y = 0.05

or, 1.6y + y = 0.05

or, 1.7y = 0.05

or, y = 0.0294 = moles of NaH2PO4

Thus, x = 0.05 - 0.0294 = 0.0206 = moles of Na2HPO4

Now, molar mass of moles of NaH2PO4 = 120 & molar mass of Na2HPO4 = 142 g/mole

Thus, mass of Na2HPO4 = moles*molar mass = 0.0206*142 = 2.9252 g

mass of NaH2PO4 = moles*molar mass = 0.0294*120 = 3.528 g

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