Lab Experiment #8:
Determining The Stoichiometry Of Chemical Reactions
A balanced chemical equation gives the mole ratios of reactants and products in a
chemical reaction. How can the stoichiometry of a chemical reaction be determined
experimentally? This experiment uses the method of continuous variations to
determine the mole ratio of the two reactants.
In the method of continuous variations, the total number of moles of reactants is kept
constant for a series of measurements. Each measurement is made with a different
mole ratio of reactants. The maximum change will occur when the mole ratio of the
reactants is closest to the optimum ratio which is the stoichiometric ratio in the chemical
equation.
An extensive property of the reaction that depends on the amount of product formed is
measured. This property may be the color intensity of a reactant or product, the mass of
a precipitate, the amount of heat produced, or the volume of a gas evolved.
The reaction we will use an example for this method is the double replacement reaction
between aqueous lead (II) nitrate and aqueous potassium iodide:
Write the balanced chemical reaction below:
Write the net ionic equation for this reaction below:
What is the IDEAL mole ratio between aqueous lead (II) nitrate and aqueous
potassium iodide based on your balanced equation above?
Look at the data table on page 3. Calculate the number of moles of each reactant
you are adding to test tubes #1-5. Record these values in the table before coming
to class. (Hint: you have volume and molarity!)
Materials
0.1 M potassium iodide, KI (aq)
0.1 M lead (II) nitrate, Pb(NO3)2 (aq)
6 test tubes
2 graduated cylinders
Pipettes
Ruler
Beakers
Procedure
1. Obtain approximately 40 mL of KI (aq) in one beaker and 40 mL of Pb(NO3)2 (aq)
in the other beaker. Make sure to label each beaker clearly.
2. In your test tubes, add the indicated volume of each reactant into test tubes #1 5. Make sure to measure the volume of each solution carefully using the
graduated cylinders.
3. After adding the reactants to each test tube, use a stirring rod to mix the
products. Let the test tubes sit for 10 minutes so the precipitate settles to the
bottom.
4. Using a ruler, measure the height of the precipitate in each test tube in
millimeters. Record the data in the table.
5. After measuring the height of each precipitate in test tubes #1 - 5, choose the
volume of Pb(NO3)2 (aq) and KI (aq) that you predict will produce the largest
amount of precipitate. Record these volumes in the data table.
6. Add your predicted volumes to test tube #6, mix with a stirring rod, let the test
tube sit for 10 minutes, and record the height of the resulting precipitate.
Test
Tube
Volume
of 0.1 M
Pb(NO3)2
(aq) (mL)
Moles of
0.1 M
Pb(NO3)2
(aq)
Volume
of 0.1 M
KI (aq)
(mL)
1
1.0
9.0
2
3.0
7.0
3
5.0
5.0
4
7.0
3.0
5
9.0
1.0
6
(you
choose)
(you
choose)
Moles of
Total
Measured
0.1 M
volume of height of
Pb(NO3)2 reactants precipitate
(aq)
(mL)
(mm)