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Phosphoric acid is a tri-protic acid, which can dissociate three different protons at three different pka’s....

Phosphoric acid is a tri-protic acid, which can dissociate three different protons at three different pka’s. These pka values are 2.13, 7.20 and 12.36.

1a. Provide the chemical equation that describes the phosphoric buffer system that exists at pH 8! Indicate the weak acid and it’s conjugate base, and the names of the components.

H2PO4- (aq) <=> H+ (aq) + HPO42- (aq)         pKa2 = 7.20

The weak acid is dihydrogen phosphate, H2PO4- (as the sodium or the potassium salt) and the conjugate base is monohydrogen phosphate, HPO42- (as the sodium or the potassium salt).

b) Calculate to 2 decimal places, the ratio of base/acid at this pH. Show all details of your working out. Include (!) in your working out the chemical formulas of the weak acid and conjugate base.

The ratio of the base/acid at pH 8.

Henderson-Hasslebach equation

pH = pKa2 + log [HPO42-]/[H2PO4-]

8 = 7.20 + log [HPO42-]/[H2PO4-]

log [HPO42-]/[H2PO4-] = 0.80

[HPO42-]/[H2PO4-] = antilog (0.80)

[HPO42-]/[H2PO4-] = 6.309 ≈ 6.31

The ratio at pH 8. is 6.31

1c. Calculate to 2 decimal places the concentration of the conjugate acid and conjugate base for a total buffer concentration of 200 mM. Show all details of your working out, including (!) the chemical formula of the acid and base, include all units and state the answer in a complete sentence.

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