Chemistry Practice Problems Equivalence Point Practice Problems Solution: Potassium sorbate, KC6H7O2 (molar mass 150 g/mol) ...

Solution: Potassium sorbate, KC6H7O2 (molar mass 150 g/mol) is commonly added to diet soft drinks as a preservative. A stock solution of KC6H7O2(aq) of known concentration must be prepared. A student titrates 45.00 mL of the stock solution with 1.25 M HCl(aq) using both an indicator and a pH meter. The value of Ka for sorbic acid, HC6H7O2, is 1.7 x 10-5.(a) Write the net ionic equation for the reaction between KC 6H7O2(aq) and HCl(aq).(b) A total of 29.95 mL of 1.25 M HCl(aq) is required to reach the equivalence point. Calculate [KC6H7O2] in the stock solution.

Problem

Potassium sorbate, KC6H7O2 (molar mass 150 g/mol) is commonly added to diet soft drinks as a preservative. A stock solution of KC6H7O2(aq) of known concentration must be prepared. A student titrates 45.00 mL of the stock solution with 1.25 M HCl(aq) using both an indicator and a pH meter. The value of Ka for sorbic acid, HC6H7O2, is 1.7 x 10-5.

(a) Write the net ionic equation for the reaction between KC 6H7O2(aq) and HCl(aq).

(b) A total of 29.95 mL of 1.25 M HCl(aq) is required to reach the equivalence point. Calculate [KC6H7O2] in the stock solution.

Solution

We are asked to determine the concentration of a stock solution of sorbic acid HC6H7O2 by first writing its net ionic reaction with HCl


Part A: Writing the net ionic equation

• Potassium sorbate, KC6H7O2 is an ionic compound which will dissociate into ions when in a solution:

          • Potassium K is from group 1A and will have +1 charge → K+

          • Sorbate is the deprotonated form of sorbic acid → C6H7O2-

• The dissociation of KC6H7O2 is:

          KC6H7O2(aq) → K+(aq) + C6H7O2-(aq)

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