Question

a. Your product research director wants you to prepare 1 liter of a buffer solution of...

a. Your product research director wants you to prepare 1 liter of a buffer solution of pH 6.5 having a buffer capacity of 0.10 using a combination of NaH2PO4 and Na2HPO4. What is the exact concentration of each constituent required for this preparation? Ka (NaH2PO4) = 6.2 x 10-8.

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Homework Answers

Answer #1

Solution:

pKa is calculated as,

pka = - log ka = - log 6.2 x 10^-8

pka = 8 - log 6.2 = 8 - 0.79 = 7.21

According to Hendersen- Hasselbalch equation,

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

6.5 = 7.21 + log [Na2HPO4] / [NaH2PO4]

log [Na2HPO4] / [NaH2PO4] = -0.71

[Na2HPO4] / [NaH2PO4] = antilog - 0.71 = 0.195 = 0.2

Thus,

[Na2HPO4] = 0.2 x [NaH2PO4]

Therefore,

For preparation of 0.10 buffer capacity,

The concentrations will be:

[Na2HPO4] = 0.2 x 0.10 = 0.02 M

[NaH2PO4] =1 x 0.10 = 0.10 M

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