A student is asked to standardize a solution of calcium hydroxide. He weighs out

Question

A student is asked to standardize a solution of calcium hydroxide. He weighs out 1.09 g potassium hydrogen phthalate (KHC8H4O4, treat this as a monoprotic acid). It requires 34.6 mL of calcium hydroxide to reach the endpoint.

A. What is the molarity of the calcium hydroxide solution? _________M

This calcium hydroxide solution is then used to titrate an unknown solution of hydrochloric acid.

B. If 16.4 mL of the calcium hydroxide solution is required to neutralize 23.0 mL of hydrochloric acid, what is the molarity of the hydrochloric acid solution? ________M

Explanation / Answer

Balanced equation:
2 KC8H5O4 + Ca(OH)2 ===> Ca(KC8H4O4)2 + 2 H2O
Reaction type: double replacement

Mass of KHP = 1.09 g  

Moles of KHP = 1.09 / 204.22 = 0.00533 Moles

Moles of Ca(OH)2 =  0.002665 Moles

Molarity of Ca(OH)2 = 0.002665 x 1000 / 34.6 = 0.0771 M

In the titration against with HCl

Moles of Ca(OH)2 = 16.4 x 0.0771 / 1000 = 0.001264

Moles of HCl =  0.001264 x 2 = 0.002529 Moles

Molarity of HCl = 0.002529 x 1000 / 23 = 0.10999 M

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