Question

You are titrating 50mL of diprotic acid H2X with standardized 0.01M NaOH solution. It takes 25 mL of NaOH to reach the first equivalence point.

a. Write down the balanced equation for each of the titration reactions

b. What is the volume of base needed to reach the second equivalence point

c. Calculate the concentration (M) of the unknown acid solution, H2X

Answer 1

a. Balanced equations,

H2X <==> HX- + H+ ........Ist dissociation

and,

HX- <==> X^2- + H+ .......IInd dissociation

b. mols of NaOH = molarity x volume = 0.01 M x 0.025 L = 2.5 x 10^-4 mols

So mols of H2X = 2.5 x 10^-4 mols

mols of NaOH required at second equivalence point = 2.5 x 10^-4 x 2 = 5 x 10^-4 mols

Volume of NaOH required to reach the second equivalence point = 5 x 10^-4/0.01 = 0.05 L = 50 mL

c. Concentration of unknown acid H2X = 2.5 x 10^-4/0.05 = 0.005 M