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 0.0400 M phosphate buffer at pH 7.460. To do this, you choose to mix the two salt forms involved in the second ionization, NaH_2 PO_4 and Na_2 HPO_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). H_3 PO_4 and NaH_2 PO_4 H_3 PO_4 and Na_2 HPO_4 H_3 PO_4 and Na_3 PO_4 Na_2 HPO_4 and Na_3 PO_4

Explanation / Answer

pKa's of phosphoric acid are 2.148, 7.198 and 12.375. If required pH is 7.460, then, 7.198 will be used. This means NaH2PO4 and Na2HPO4 will be used.

pH = pKa + log ([A-]/[HA])

Now, for A- you put Na2HPO4 concentration, and for HA you put NaH2PO4 concentration:

7.460 = 7.198 + log (A/HA)

log(A/HA)= - 0.262

A/HA = 0.5816M

0.0400M

The fold difference between these salts

HA+A=0.0400M

A/HA= 0.5816M

HA=0.026M

A=0.014M

This means, we need to put 0.026 moles of NaH2PO4 and 0.014 moles of NaHPO4 salts into 1 liters of solution.

Molar mass of NaH2PO4 = 119.977g/mol

Weight of NaH2PO4 = 119.977 x 0.026 = 3.12 g

Molar mass of Na2HPO4 = 141.96 g/mol

Weight of Na2HPO4 = 119.977 x 0.014 = 1.68 g

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