1 Rev 9/2020

Experiment #3:

The Determination of an Equilibrium Constant Expression

Lab Instructions

• This lab is a hybrid lab. There will be virtual and onsite portions

• Each student will need to do the following individually: 1. Collect data from the video provided (pages 4-5) 2. Calculations for the data provided (pages 4-5) 3. Perform 4 titrations (page 6)

Required Skills

To perform this experiment, you will need to have mastered all of the skills from Experiment #1

and also be able to properly …

• Obtain solid samples.

• Weigh solid samples.

• Perform a vacuum filtration.

• Perform a gravity filtration.

• Perform a titration.

You should thoroughly review this material in the basic skills document available on the Moodle

site. Remember, that the instructor and TAs are constantly assessing your level of preparation.

Students who are disorganized, confused, and uninformed will receive lower laboratory scores.

Background

This laboratory experiment is focused on determining the equilibrium constant expression

for the solubility of an ionic solid. We will consider the process of solid calcium iodate converting

into aqueous calcium and iodate ions…

Ca(IO3)2(s) ? Ca2+(aq) + 2IO3- (aq) (Rxn 1)

When a system reaches chemical equilibrium the concentrations of the chemical species

become constant with respect to time. Furthermore, it has been found that specific ratios of the

chemical concentrations raised to precise exponential powers are always the same (no matter

what the starting concentrations were). The magnitude of the correct ratio is called the

equilibrium constant (often denoted as K) and the mathematical relationship linking the

concentrations to the constant is called the equilibrium constant expression. You will empirically

determine the equilibrium constant expression for Rxn 1 in this laboratory experiment.

Experiment #3: Determination of an Equilibrium Constant Expression

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In order to narrow the focus, you will set out to determine which of the following ratios of concentration is best used as the equilibrium constant expression for Rxn 1…

a. K = ([Ca2+][IO3-])/[Ca(IO3)2] b. K = [Ca2+] [IO3-] c. K = [Ca2+]2 [IO3-] d. K = [Ca2+] [IO3-]2

Your careful experimental work will allow you to answer this multiple-choice question with

certainty.

The laboratory can be broken down into two main parts. In part I, you will first make

known quantities of two of the pertinent species, Ca2+ and Ca(IO3)2. In addition, you will

determine the concentration of thiosulfate in a solution that will serve as your titrating agent. In

part II, you will make eight mixtures with different starting amounts of Ca2+ and Ca(IO3)2 and allow

them to equilibrate. You will then perform a series of titrations to determine the equilibrium

concentrations of Ca2+ and IO3-. And finally, you will use the concentration and titration data to

experimentally determine the correct equilibrium constant expression and the equilibrium

constant.

Experiment #3: Determination of an Equilibrium Constant Expression

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Procedure – Part I Chemicals Needed for Part I:

Goal Molarity Formula Name Quantity

#1 CaCl2•2H2O(s)

Calcium Chloride Dihydrate

Determined in prior to class calculations

#1 KIO3(s)

Potassium Iodate

Determined in prior to class calculations

#2 CaCl2•2H2O(s)

Calcium Chloride Dihydrate

Determined in prior to class calculations

#3 Unknown Na2S2O3(aq) Sodium Thiosulfate 100 mL

#3 0.01XX M KIO3(aq) Potassium Iodate 50 mL

#3 KI(aq) Potassium Iodide 40 mL

#3 1.0 M HCl Hydrochloric Acid 20 mL

#3 0.20% Starch Solution 5 mL

• Break your team up. One will work on Goal #1; one will work on Goal #2 and the other

members will work on Goal #3.