Question

Phosphoric acid is a triprotic acid with the following pKa values:

pka1=2.148

pka2=7.198

pka3=12.375

You wish to prepare 1.000 L of a 0.0500 M phosphate buffer at pH 7.780. To do this, you choose to mix the two salt forms involved in the second ionization, NaH2PO4 and Na2HPO4, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture?

Answer 1

The molar ratio of the components of the buffer is calculated, using the henderson hasselbach equation:

i) n Salt / n Acid = 10 ^ (pH-pKa) = 10 ^ (7.78 - 7.198) = 3.82

It has:

ii) n Salt + n Acid = M * V = 0.05 M * 1 L = 0.05 mol

System of equations between i and ii is applied:

n Salt = 0.04 mol

n Acid = 0.01 mol

The mass of each compound to be added is calculated:

m Na2HPO4 = n * MM = 0.04 * 142 = 5.68 g

m NaH2PO4 = 0.01 * 120 = 1.2 g

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