Experiment 9: Determination of Mass and Mole Relationship in a Chemical Reaction
Background: This experiment uses the concept of continuous variation to determine mass and
mole relationships. Continuous variation keeps the total volume of two reactants constant, but
varies the ratios in which they combine. The optimum ratio would be the one in which the
maximum amount of both reactants of known concentration are consumed and the maximum
amount of product(s) is produced. Since the reaction is exothermic, and heat is therefore a
product, the ratio of the two reactants that produces the greatest amount of heat is a function of
the actual stoichiometric relationship. Other products that could be used to determine actual
molar relationships might include color intensity, mass of precipitate formed, amount of gas
evolved, and so on.
Scenario: The active ingredient in commercial bleach is NaClO (see Experiment 8). A student
was given a sample of commercial bleach which was labeled 5.20 % NaClO by mass. The stu-
dent was then directed to prepare 300. mL of a 0.500 M NaClO solution. The student was also
directed to prepare 300. mL of a 0.500 M solution of sodium thiosulfate, Na 2 S 2 O 3 in 0.200 M
sodium hydroxide. Both solutions were then allowed to reach a room temperature of 25.0°C.
Keeping a constant final volume of 50.0 mL of solution, the student mixed various amounts of
sodium hypochlorite and sodium thiosulfate together, stirring well, and recorded the maximum
temperature (±0.2°C) of the solution with a calibrated temperature probe. The change in tem-
perature (∆T) is equal to Tfinal−25.0°C. A chart and graph of the data is given in Figure 1.
Figure 135
30
25
20
15
10
5
0
1234567
Trial NumberTemperature Change°CTemperature Change vs. Volume Ratios8 9 10 11Trial # ∆T°C mL NaClO mL Na 2 S 2 O 3 vol ratio
10 0 50 x
2 3.9 5 45 1:9
3 8.1 10 40 1:4
4 11.3 15 35 1:2.3
5 14.7 20 30 1:1.5
6 18.8 25 25 1:1
7 22.6 30 20 1.5:1
8 27.6 35 15 2.3:1
9 31.9 40 10 4:1
10 12 45 5 9:1
11 0.1 50 0 xLaboratory Experiments