Question

A pH 3.30 buffer is prepared from iodoacetic acid ICH₂COOH (pK_a = 3.175), and sodium iodoacetate, ICH₂COO^-Na⁺ . If ionic strength of the buffer solution must not exceed 0.200 M, what are the maximum concentrations of the two components that could be used?

What volume of KOH (conc = 1.850 M), could be added to 50.00 mL of the buffer solution of part (a), before the pH would increase to 4.000?

Answer 1

ICH2COONa + ICH2COOH = 0.200 M

pH = pKa + log [salt/acid]

3.30 = 3.175 + log (ICH2COONa/ICH2COOH)

1.33 = ICH2COONa / ICH2COOH

ICH2COONa + ICH2COOH = 0.200 M

1.33 ICH2COOH + ICH2COOH = 0.200

2.33 ICH2COOH = 0.200

[ICH2COOH] = 0.0858 M

ICH2COONa = 1.33 ICH2COOH = 1.33 x 0.0858

                      = 0.114 M

[ICH2COONa] = 0.114 M

ICH2COOH     +    KOH -------------------> ICH2COOK + H2O

0.0858 x 50         1.850 V                             0                      0

4.29 - 1.850 V           0                                   1.850 V          1.850V

here salt and acid remaining so it forms acidic buffer.

pH = pKa + log [salt / acid]

4.00 = 3.175 + log [1.850V / 4.29 - 1.850V]

6.68 = [1.850V / 4.29 - 1.850V]

V = 2.02 mL

volume of KOH added = 2.02 mL