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Buffer preparation is a necessary part of working with any living cell culture. Buffers are most...

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Chemistry

Buffer preparation is a necessary part of working with anyliving cell culture. Buffers are most commonly prepared by startingwith a weak acid, HA, or weak base, A-, and adding enough strongacid, H+, or strong base, OH-, to find the correct ratio of[HA]/[A-]. A pH = 7.3 buffer is needed in the lab. This buffer ismade by first dissolving 52.25 g K2HPO4 in 400 mL of water. What isthe pH of this salt solution? This solution of course will be toobasic becuase we only have the base of the buffer present. Whatmust the value of the ratio of [H2PO4-]/[HPO42-] be to create abuffer solution that is pH = 7.3? How many moles of HCl must beadded to the 400 mL solution of HPO42-(aq) to prepare the pH = 7.3buffer solution? How many mL of 1 M HCl must be added to thesolution to create the pH = 7.3 buffer solution?

Answer & Explanation Solved by verified expert
3.9 Ratings (609 Votes)

a)

moles of K2HPO4- = 52.25 / 174.17 = 0.3

concentration of HPO42- = 0.3 x 400 / 1000 = 0.12 M

HPO42- + H2O   ----------------> H2PO4- + OH-

0.12                                                   0             0

0.12 - x                                               x             x

Kb2 = x^2 / 0.12 - x

1 x 10^-14 / 6.2×10–8 = x^2 / 0.12 - x

x = 1.39 x 10^-4

[OH-] = 1.39 x 10^-4 M

pOH = -log [OH-] = -log(1.39 x 10^-4)

         = 3.86

pH = 10.14

pH = pKa2 + log [salt / acid]

7.3 = 7.21 + log [HPO42-]/[H2PO4-]

[HPO42-]/[H2PO4-] = 1.23

[H2PO4-]/[HPO42-] = 0.813
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