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Being able to control the pH, is essential in biochemical experiments. (i) Predict the pH region...

Being able to control the pH, is essential in biochemical experiments. (i) Predict the pH region in which each of the following buffers will be effective, assuming equal molar concentrations of the acid and its conjugate base: (a) sodium lactate and benzoic acid. (b) sodium benzoate and benzoic acid. (c) hydroxylamine and hydroxyl ammonium chloride. (ii) Draw a plot of the pH curve of a solution containing 0.10 M NaCH3CO2(aq) and a variable amount of acetic acid.

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Answer #1

Ans i:

pH of acidic buffer = pKa + log ([conjugate base]/[Acid])

pH of basic buffer = 14 – {pKb + log ([conjugate acid]/[base]) }

Buffer (Acid:Conjugate base = 1M:1M)

pKa of Acid/pKb of base

(At 25oC)

Effective pH region

Around

a.

Sodium lactate + Benzoic acid

Benzoic acid = 4.20

4.20

b.

Sodium benzoate + Benzoic acid

Benzoic acid = 4.20

4.20

c.

Hydroxylamine + Hydroxyl ammonium chloride

Hydroxylamine = 7.97

6.03

Ans ii:

pH curve of a solution containing 0.10 M CH3CO2Na (aq) (sodium acetate) and a variable amount of acetic acid.

pH of acidic buffer = pKa + log ([conjugate base]/[Acid])

pKa of CH3COOH = 4.756

CH3CO2Na conc.

CH3COOH conc.

pH

1.

0.10 M

0.02 M

5.455

2.

0.04 M

5.154

3.

0.06 M

4.978

4.

0.08 M

4.853

5.

0.10 M

4.756

6.

0.12 M

4.677

7.

0.14 M

4.610

5.6 5.4 5.2 H 4.8 4.6 4.4 4.2 4 0.02 M 0.04 M 0.06 M 0.08 M 0.10 M 0.12 M 0.14 M

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