You are asked to prepare several different buffers. Given below is a list of ava

Question

You are asked to prepare several different buffers. Given below is a list of available materials. For simplicity, you may neglect volume changes which occur upon mixing any two liquids or dissolving any solids. In each ease, your buffer should have a reasonable buffering capacity. The buffer component present in the lower concentration (whether it be the acid or base) should be present at exactly 0.100 M (so, the other component must be at that concentration or higher). Design a recipe meeting these criteria to produce 100.0 mL of each of the buffers required below. (It is sufficient to calculate volumes and masses needed; you need not provide the technician with detailed instructions on weighing procedure, glassware used, etc..) Available Solids: benzoic acid (C_6H_5COOH), sodium chlorite (NaClO_2), potassium hydrogen phosphate trihydrate (K_2HPO_4 middot H_2O) Available Solutions: 0.1973 M HCl, 0.1309 M NaOH, 2.142 M ammonia Buffers required: (A) pH 4.00 (B) pH 7.00 (C) pH 10.00

Explanation / Answer

For buffer with pH,

(A) pH = 4

benzoic acid with NaOH will be used

Using Hendersen-Hasselbalck equation,

pH = pKa + log(base/acid)

let x moles of base is added

4 = 4.2 + log(x/0.1 M x 0.1 L - x)

0.0063 - 0.631x = x

x = 0.004 moles of NaOH needed

Volume of NaOH needed = 0.004 mol x 1000/0.1309 M = 30.56 ml

(B) pH = 7.00

K2HPO4.3H2O with HCl would be used

let x moles of HCl is added

7 = 7.2 + log(0.1 M x 0.1 L - x/x)

0.631x = 0.01 mol - x

moles of HCl = x = 0.006 mol

Volume of HCl needed = 0.006 mol x 1000/0.1973 M = 30.41 ml

Similarly for pH 10 calculations can be done

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