DETERMINATION OF REACTION STOICHIOMETRY
Purpose:
The purpose of this experiment is to determine the mole ratio of reactants in a chemical reaction.
You will use the method of continuous variations, in which you mix solutions of known concentration,
keeping the total volume constant but varying the ratios of reactant solutions. The optimal ratio, in which
the reactant ratio is stoichiometric, will give the maximum yield of product.
How should you measure the product yield? Depending on the nature of the product, you could determine
the mass of a precipitate, or volume of gas, or color intensity. This reaction is exothermic, so you can
monitor energy as a product. If the total volume of the reaction mixture is constant, the energy produced
will be directly proportional to the temperature change.
We will label reactants A and B. Both solutions are 0.50 M. 0.5M NaClO is prepare by diluting 652mL
of 6.00% Chlorax bleach or 745 mL of Generic of 5.25% in 1.0L of water. Prepare 4 L per lab section.
124g Na2S2O3 x 5H2O or 79.1g anhydrous Na2S2O3 in 500mL distilled water. Add 8 g NaOH stir and
dissolve. Dilute mix to 1.0L. Make 2 L per section.
Procedure
Safety: Both solutions are significantly basic, and therefore harmful to skin and eyes. If you spill them,
rinse with LOTS of cold water. GOGGLES, of course. One of the solutions is laundry bleach, so don't
wear anything you really care about (unless it’s already white?).
1. Label two 50 mL cylinders A and B to measure larger volumes, two 10 mL graduated pipets A and B
to measure smaller volumes. Use a clean dry foam cup for your reaction beaker.
2. Obtain 100 mL of solution A and 100 mL of solution B in clean, dry beakers. Both solutions should
be at the same temperature; check this (rinse and dry the thermometer thoroughly between solutions to
avoid contamination) and note the result in your journal. If they are different, consult the instructor to
find out how to analyze your data.
3. You have been assigned three combinations of A and B to test. Put the first assigned volume of
solution A into the foam cup and add the first assigned volume of solution B. Stir and record the
highest temperature reached. Pour the reaction mixture down the drain, rinse and dry the cup and
thermometer, and repeat the procedure with the other assigned combinations. Note that the total
volume is always 50 mL. Report the volume combinations tested and the resulting ∆T on the class
data sheet.
Analysis
Obtain a copy of the class data and tape it into your journal. Plot
∆T on the ordinate and the volume ratio on the abscissa.
ĘT
Draw two straight lines that best fit the data and determine the
point where they intersect (maximum ∆T). From the intersection
point, determine the optimal (stoichiometric) mole ratio
of A to B. Report your ratio on the class results.
mL A
0
10
20
30
40
50
mL B
50
40
30
20
10
0
Discussion
1. Use the optimal ratio to fill in the coefficients of the chemical equation:
___ A + ___ B → products
2. Briefly discuss the following aspects of the experimental design:
a. how the method of continuous variations works, using the idea of limiting reagent.
b. why you did not need to convert your volumes of A and B to moles, even though equations are
written in moles.
c. why a graphical analysis is more accurate than just using the ratio with the maximum temperature
change.
3. The coefficients must be a whole number ratio. Did we obtain a whole number ratio, within
experimental error? Discuss possible sources of error in this procedure.
Team
combination 1 combination 2 combination 3
A
B
A
B
A
B
45.0
5.0
25.0
25.0
10.0
40.0
35.0
15.0
22.5
27.5
12.5
37.5
40.0
10.0
15.0
35.0
20.0
30.0
32.5
17.5
5.0
45.0
27.5
22.5
30.0
20.0
12.5
37.5
25.0
25.0
37.5
12.5
20.0
30.0
10.0
40.0
15.0
35.0
32.5
17.5
40.0
10.0
45.0
5.0
22.2
27.5
12.5
37.5
37.5
12.5
5.0
45.0
22.5
27.5
30.0
20.0
15.0
35.0
17.5
32.5
35.0
15.0
17.5
32.5
10.0
40.0
27.5
22.5
5.0
45.0
37.5
12.5
20.0
30.0
32.5
17.5
25.0
25.0
17.5
32.5
30.0
20.0
40.0
10.0
27.5
22.5
35.0
15.0
45.0
5.0