Titrimetic Analysis of Mixtures

Shown below is a titration curve for a mixture of two monoprotic weak acids, HA and HB, where pKa,HA = 4.00 and pKa,HB = 8.00.  The titrant is 0.200 M NaOH.

(a) Briefly explain how you can tell that this is the titration curve for a mixture of weak acids and not the titration curve for a diprotic acid, such as H2A.

(b) Supplement your answer to part (a) by sketching the expected titration curve for the diprotic weak acid H2A (for simplicity just superimpose your sketch on the titration curve for the mixture). Assume that pKa1 = 4.00, pKa2 = 8.00 and that the concentration of H2A is the same as that for HA in the titration curve shown above. Don’t worry about accurately estimating the pH levels after the second equivalence point.

(c) Using the titration curve, estimate the concentration of the weaker of the two acids (be sure to show you arrived at the volume of NaOH used in this part of the titration).

(d) The sample being titrated was prepared by adding 0.311 g of HA and 0.258 g of HB to 50.00 mL of water. Estimate the equivalent weight of HA.

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