Question

An aqueous solution containing 1.00 g of oxobutanedioc acid (FM 132.07) per 100 mL was titrated with 0.09432 M NaOH.

1. Calculate the pH at th following volumes of added base: 0.5V_e1, V_e1, 1.5V_e2, V_e2, 1.05V_e2
2. Sketch the titration curve, using the values calculated above.
3. Which equivalence point would be best to use in this titration (which one is not blurred)?

Ka1 = 2.56

Ka2 = 4.37

Ve1 = equivalence point 1

Ve2 = equivalence point 2

Answer 1

Lets say oxobutanedoic acid = H2A (a weak diprotic acid)

H2A + NaOH   NaHA + H2O

NaHA + NaOH Na2A + H2O

1. as we know, at first half equivalence point , a buffer solution is formed with maximum buffer capacity, whose pH is given as : pH = 0.5pKa1 = 0.5 2.56 = 1.28

2. at first equivalence point a solution of an amphiprotic salt NaHA is formed,

whose

3. at second half equivalence point , a buffer solution is formed with maximum buffer capacity, whose pH is given as :

pH = 0.5pKa2

pH = 0.5 4.37 = 2.185

4. at second equivalence point , A solution of salt Na2A is formed, whose pH is given by

where C = concentartion of salt = mole of Limiting reagent / total volume at second equivalence point = 0.029

5. at 1.05 Ve2 , an acidic buffer (NaHA and Na2A) is formed

whose pH is given as

pH = 1.3