Archer
29 September 2011
Finding the Ratio of Moles of Reactants in a Chemical Reaction
Purpose: The purpose of this lab is to find the ratio of the reactants. This can be done by measuring the
heat released by different ratios of substances mixed together then calculating the mole ratio from the
graph of ratio to heat released. The significance of this lab is that it’s another means of finding
unknowns. When the mole ratio is known, the substance can then be used with other methods to find
the identity of the substance.
Hypothesis: The hypothesis is that the mole ratio of the reactants can be found by measuring the heat
released and estimating the ratio of volume in which the maximum heat is released which then can be
used to find the mole ratio. The maximum heat release means that the highest amount of reaction
occurred. This means that nearly all of the reactants are involved in the reaction thus it shows the ratio
of the reactant. If the ratio of the reactants is wrong, less reactions would occur and so less heat would
be released.
Materials:
Materials
0.5 M NaClO
0.5 M Solution C (Na2SO3)
Calorimeter
Thermometer
50-mL graduated cylinder
25-mL graduated cylinder
200-mL graduated cylinder
600-mL beaker
Labeling tape
Eye dropper
Paper towel
Procedure:
1.) Measure 250mL of NaClO
2.) Transfer it to a 600-mL beaker
3.) Label the beaker
4.) Measure 160mL of solution C (Na2SO3)
5.) Pour it into another 600-mL beaker
6.) Label the beaker
7.) Measure the temperature of each solution
8.) Measure 5mL of NaClO
9.) Transfer it into a calorimeter
10.) Put the thermometer through the calorimeter’s cap
Quantity
241mL
159mL
1 calorimeter
1 thermometer
2 cylinders
2 cylinders
2 cylinders
2 beakers
1 roll
2 droppers
1 roll
Archer
11.) Measure 45mL of solution C (Na2SO3)
12.) Add it to the calorimeter
13.) Wait until the red alcohol start going down
14.) Record the highest temperature
15.) Repeat step 8-14 for the ratio of 15 to 35, 25 to 25, 35 to 15, 37.5 to 12.5, 38.5 to 11.5, 40 to 10,
and 45 to 5
Results: The NaClO smell like the chlorine of a swimming pool. NaClO easily created bubbles. NaClO has
a slight yellow-green color while solution C (Na2SO3) seemed to be completely colorless and odorless.
NaClO is somewhat slippery while solution C (Na2SO3) is not.
Initial Temperature (°C)
NaClO
27
Solution C (Na2SO3)
27
Ratio of Volume of Solution to Heat Released
Solution C (Na2SO3)
NaClO (mL)
Tfinal (°C)
(mL)
5
45
28
15
35
30.1
25
25
33
35
15
34.9
37.5
12.5
36.5
38.5
11.5
35.3
40
10
32.7
45
5
29
Mole Ratio
3:1
ΔT (°C)
1
3
6
8
10
8
6
2
Archer
Ratio of Volume of Solution to Heat Released
40
35
Temperature (oC)
30
25
20
15
Heat Released
10
5
0
0
10
50
40
20
30
Volume of NaClO (mL)
30
20
Volume of Solution C (mL)
40
50
10
0
Analysis:
ΔT = (Tfinal) – (Tinitial)
Change in temperature equals to temperature final minus temperature initial
28 – 27 = 1 °C
30.1 – 27 = 3.1 °C
33 – 27 = 6 °C
34.9 – 27 = 7.9 °C
36.5 – 27 = 9.5 °C
35.3 – 27 = 8.3 °C
32.7 – 27 = 5.7 °C
29 – 27 = 2 °C
Moles of solution = (Volume of solution) × (Molarity of solution)
Moles of solution equals volume of solution times the molarity of solution
0.0375 × 0.5 = 1.875 × 10-2 mole of NaClO
Archer
0.0125 × 0.5 = 6.25 × 10-3 mole of Solution C (Na2SO3)
Mole ratio = (Ratio of the highest temperature change)
Mole ratio = (Moles of solution) / (Moles of the smallest number)
(1.875 × 10-2) / (6.25 × 10-3) = 3 moles of NaClO
(6.25 × 10-3) / (6.25 × 10-3) = 1 mole of Solution C (Na2SO3)
There should not be any mistake about the hypothesis. If the temperature change is small, it meant that
one of the reactant is limiting the reaction so only a small part of the solution reacts thus releasing only
a small amount of heat. The more heat released meant that the more reaction occurred. Thus, the
highest heat release meant that the highest amount of reaction occurred and nearly all of the reactants
had reacted and turned into the product. Many attempts were done so that the trend of the reaction
can be known and the peak of the reaction could be determined. The total volume of the solutions was
kept constant so that heat change depends only on the ratio of the solution. It is not necessary that the
concentrations of the two solutions are the same, but the different concentration need to be taken into
account when calculating for the ratio. Thus, having the same concentration of the reactants made the
calculations easier. The limiting reagent is the reagent or reactant that limits the reaction. The limiting
reagent caused the reaction to stop because the reagent had all been turned into product even though
the other reagent had not been used up. The temperature limits the experiment precision because if the
temperature was wrong, then a wrong peak could have been used causing the ratio to be wrong. In the
upward sloping line, the NaClO was the limiting reagent while the solution C (Na2SO3) was the limiting in
downward sloping line. In the case that the reaction produced a precipitate or gas, the amount of
precipitate or gas formed or the amount of reactant left could be used in the method of continuous
variation. It is more accurate to use the point of intersection of the two lines to find the mole ratio
because the greatest temperature change could be because of some factors other than the reaction
where as the point of intersection is found from the average of the ratios attempt. If two solutions are
not at the same initial temperature, the average of the initial temperature must be found as well as the
maximum temperature must be subtracted by the difference in the initial temperature.
Conclusion: The hypothesis cannot be proven true. This is because the solution C, which is Na2SO3, gave
found by this experiment gave the incorrect ratio. The ratio of NaClO to Na2SO3 should have been one to
one but the ratio found from this experiment was three to one. There could be several errors in during
course of the experiment. The peak from the result that was used to calculate the ratio could be wrong.
The ratio of the solution that was tried was done in an insufficient amount of attempt to be confirmed
that it was the peak. This would cause the ratio calculated from it to be wrong. Another error could be
that the droppers were not labeled. This could have caused the droppers to be swapped during the
course of the experiment. This would cause the ratio of the solutions actually used to be different from
the data took from the experiment. In the future, these errors could be prevent by doing more attempts
to make sure of the peak. Also, all equipments should be labeled so that it would not get mixed up.
Archer