Question

You need to prepare a buffer for biochemistry lab. The required solution is 0.5Msodium phosphate, pH 7.0. Use the Henderson-Hasselbalch equation to calculate the number of moles and grams of moobasic sodium phosphate ( NaH2PO4) and dibasic sodium phosphate ( Na2HPO4) necessary to make 1 liter of solution. The pKa for this buffer is 7.21

Answer #1

Using Hendersen-Hasselbalck equation,

pH = pKa + log(base/acid)

feed the values given,

7.0 = 7.21 + log([Na2HPO4][NaH2PO4])

[Na2HPO4] = 0.616[NaH2PO4]

[Na2HPO4] + [NaH2PO4] = 0.5

0.616[NaH2PO4] + [NaH2PO4] = 0.5

[NaH2PO4] = 0.31 M

[Na2HPO4] = 0.19 M

For a 1 L solution,

Molarity = moles/L = moles

Thus,

moles of Na2HPO4 needed = 0.19 mols

moles of NaH2PO4 needed = 0.31 mols

and,

grams of Na2HPO4 needed = 0.19 mols x 141.96 g/mol = 44.01 g

grams of NaH2PO4 needed = 0.31 mols x 119.98 g/mol = 37.94 g

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