Question

НООС СООН CEC H Η Maleic Acid PH 20 40 60 Volume of NaOH (mL) The curve above shows the titration of 250 mL of 100 mM maleic acid using 1 M NaOH. a) How many titratable protons are present in this acid? [1 marks]. b) Indicate on the curve with a "O" the end point titrations of each titratable group. [1 mark] c) Indicate on the titration curve with a clear "X" each pKa value associated with these titratable protons and estimate to the nearest 0.5 unit the value for each pka. [3 marks]

Answer 1

a) Maleic acid has two COOH groups which can give protons. Hence the number of titratable protons is two

b)

For the first titratable proton,

250 ml of 100 mM of Maleic acid = x ml of 1 M NaOH

Therefore x = 250 *0.1/1 = 25 ml

Volume of NaOH for first titratable proton = 25 ml

For the second titratable proton,

250 ml of 100 mM of Maleic acid = y ml of 1 M NaOH

Therefore y = 250 *0.1/1 = 25 ml

Total volume of NaOH consumed for both the titratable protons = 50 ml

Therefore, the end points are marked in the following figure:

14 12 10 8 6 4- 2 20 40 Volume of NaOH (mL) 60 нd

c)

pK_a of a weak acid is the region where in the pH does not change appreciably with small addition of base, due to buffer action of the acid. These are the plateau regions marked by X.

14 12- 10 X- 2 20 60 40 Volume of NaOH (mL) Hd

From the graph,

pKa1 = 2

pKa2 = 6