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

Question

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 7.50. You add 15.0 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.

Explanation / Answer

we will use hendersen equation for solving the question

Which is

pH = pKa + Log[salt/acdi]

NaH2PO4 = weak acid = mass / mol wt X volume = 15 / 120 X 1 = 0.125 M
Na2HPO4 = Salt = ??
pKa = 7.21 ( dibasic acid)
7.5 = 7.21 + log ( salt / 0.125)
     0.29 = log(salt / 0.125)

taking antilog

1.94 = [salt] / 0.125

so [salt] = 0.2425M

molarity = mass / mol wt X volume

0.2425 = mass / 142 X 1

so mass = 34.43 g will be required for preapartion of buffer.

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