Phosphoric acid undergoes deprotonation in three steps to form inorganic phospha
ID: 536334 • Letter: P
Question
Phosphoric acid undergoes deprotonation in three steps to form inorganic phosphate (phosphate ion). The pKa values for phosphoric acid are: pKa1 2.12 pKal 7.21 pKo1 12.67 The total concentration of phosphate in human blood (pH 7.4) is approximately 1.00 mM (millimolar, 1 mmol o.001 mol). A scientist working with human blood in an experiment needs 1.00 liter of 1.00mM phosphate buffer with a pH of 7.4. The lab has on hand: 3.00 M NaOH 6.00 M H3PO4 NaH2PO4 solid 3.00 M HCl Na2HPO4 solid ddH20 (distilled, deionized water) List the amounts/volumes of the components necessary to make the buffer. Hint: You may not need to use all the chemicals listed. You may construct a more concentrated solution and then dilute it. There are many correct answers to this problem; you just need to present one.Explanation / Answer
Preparation of buffer,
Using solid NaH2PO4 and solid Na2HPO4
Hendersen-Hasselbalck equation,
pH = pKa + log(base/acid)
pH = pKa2 + log(Na2HPO4/NaH2PO4)
7.4 = 7.21 + log(Na2HPO4/NaH2PO4)
(Na2HPO4) = 1.55(NaH2PO4)
(NaH2PO4) + (Na2HPO4) = 1 mM x 1 L = 1 mmol
(NaH2PO4) + 1.55(NaH2PO4) = 1 mmol
(NaH2PO4) = 1 mmol/2.55 = 0.39 mmol
mass of solid NaH2PO4 needed = 0.39 x 10^-3 mol x 119.98 g/mol = 0.0468 g
(Na2HPO4) = 1 - 0.39 = 0.61 mmol
mass of solid Na2HPO4 needed = 0.61 x 10^-3 mol x 141.96 g/mol = 0.0866 g