Question

Many of the biochemistry experiments and cell cultures need to be pH buffered in a narrow...

Many of the biochemistry experiments and cell cultures need to be pH buffered in a narrow range. Many different buffers are used, but phosphate buffers are a classic choice. Suppose you need to make a 1 liter phosphate buffer solution with a pH of 6.70. You add 25.0 grams of sodium dihydrogenphosphate (NaH2PO4) to a 1.00 liter flask. How many grams of sodium hydrogen phosphate (Na2HPO4) do you need to add to obtain the desired pH buffer solution? The pKa values for the three ionization states of phosphoric acid are 2.16, 7.21 and 12.32. The exacting nature of these types of solutions requires that you be within 0.25 grams to receive credit for this question.

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Answer #1

NaH2PO4 mass = 25 g

NaH2PO4 molar mass = 119.977 g/mol

moles of NaH2PO4 = 25 / 119.977

                                = 0.208

pH = 6.70

pKa2 =7.21

pH = pKa2 + log [Na2HPO4]/[NaH2PO4]

6.70 = 7.21 + log [x /0.208]

-0.51 = log [x /0.208]

0.309 = x /0.208

x = 0.06427 moles

Na2HPO4 molar mass = 141.9588 g /mol

mass = moles x molar mass

          = 0.06427 x 141.9588

          = 9.12g

mass of Na2HPO4 needed = 9.12 g

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