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Use the Henderson - Hasselbalch equation to calculate the proprotion in which two equal concentrations of...

Use the Henderson - Hasselbalch equation to calculate the proprotion in which two equal concentrations of H2PO4- ( acid) and HPO42- (conjugate base) should be mixed to produce a phosphate buffer at a pH of 7. The pKa for this acid-base pair is 6.86.

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Answer #1

According to Henderson- hasselbalch equation:

pH = pKa + Log [conjugate base]/[weak acid]

The equilibrium reaction of these two species would be :


H2PO4- => H+ + HPO42-

So when H2PO4- has given up an hydronium ion the left over part is HPO4 2-.

Given pH = 7, pKa = 6.86
Now calculate the ratio of conjugate base to weak acid using the henderson hasselbalch equation:

pH = pKa + Log [conjugate base]/[weak acid]

pH = pKa + Log [HPO4 2-]/[H2PO4-]

7 = 6.86 + log [HPO4 2-]/[H2PO4-]
7 - 6.86 = log [HPO4 2-]/[H2PO4-]
0.14 = log [HPO4 2-]/[H2PO4-]

To get rid of the log lets take the anti log of both sides:

10^0.14 (this means 10 to the power of -0.2) = [HPO4 2-]/[H2PO4-]
1.38 = [HPO4 2-]/[H2PO4-]

This means we need only1.38 times the of the amount of HPO4 2- than of H2PO4-.

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