Question

The titration of Maleic Acid

HOOC COOH CC Maleic Acid Hd 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

Part a) In Maleic acid, there are two titratable protons which are associated with the –COOH group.

Part b) At the endpoint, there should be a sharp change in pH. The endpoint is indicated with the “O” mark in the picture uploaded below.

Part c) We have Henderson- Hasselbach equation,

pH = pKa + log[conjugate base]/[acid]

At the first equivalence point, [conjugate base] = [acid]

Therefore,

pH = pKa1 = 2.1

At the first endpoint (O1), one proton will be deprotonated and AH- will be left.

Therefore, pH = 1/2 (pKa1 + pKa2) = 4.3

At the second equivalence point also, [conjugate base] = [acid]

Therefore,

pH = pKa2 = 6.5

At the second endpoint (O2), both protons will be deprotonated and only A^2- is left.   

Therefore, pH = 9.5   

14 12 10 pH 9.5 = 8 pH-6.5 6 pH 4.3 4- XpH 2.1 2 20 40 Volume of NaOH (mL) 60 Hа