Determining the Stoichiometry of Chemical Reactions
Experiment No. 2
Toby Johnson
AP. Chemistry
Clarion Limestone High School
February 9, 2016
Abstract:
After two precise observations and tests I was able to find the optimum mole ratio for the
formation of a precipitate in a double replacement reaction. In the first test I measured out the
approximate volume of the iron (III) nitrate solution to each of the graduated cylinders and
then measured out the approximate volume of sodium hydroxide to each of the graduated
cylinders. These measurements were based off the given data table one. For the second test I
measured out the approximate volumes of both copper (II) chloride and sodium phosphate
solutions to each of the seven graduated cylinder. These measurements were based off data
table two. After combining both solutions in each test, I then stirred them both and observed
the precipitate given off afterwards. Based on the given values shown in both tests by the
mixing of the reactants, I was able to have these volumes plotted versus the mole ratio for both
of these double replacement reactions.
Apparatus:
For each of these two tests I used graduated cylinders to measure out each of the amounts
required from both data tables. I used both 50-mL and 100-mL graduated cylinders. These
helped with measuring the approximate volumes required for these reactions and with being
able to record the volume of precipitate given off from each reaction in each of the 100-mL
graduated cylinders.
Theory:
For test one knowing that the Iron in Iron (III) Nitrate acts a Lewis acid in the solution and when
combined the precipitate formed remains insoluble as long as the Iron (III) Nitrate is not in
excess of the stoichiometric mole ratio. Based on this, I knew that mixing both Iron (III) Nitrate
and Sodium Hydroxide will leave a precipitate that will dissolve since the Iron (III) Nitrate is in
excess and be able to record the volume of the precipitate given off. In test two the Copper (II)
in the Copper (II) Chloride acted the same way with the Sodium Phosphate giving a precipitate
that dissolves. Based on the precipitates left behind and recorded I was able to plot both data
given off by these two double replacement reactions versus the mole ratio.
Procedure:
Starting the experiment off for test one I first got two 50-mL and seven 100-mL graduated
cylinders and made sure they were clean and not contaminated from previous use. After
making sure they were cleaned out properly, I then labeled each of the seven graduated
cylinders one through seven. I then used one of 50-mL graduated cylinder to add the
approximate volume of Iron (III) Nitrate to each of the 100-mL graduated cylinders based off
the data shown in data table one. Once I added the approximate volume of Iron (III) Nitrate, I
then used the second 50-mL graduated cylinder to add the approximate volume of Sodium
Hydroxide to each of the 100-mL graduated cylinders given in data table one. After each of
these volumes were added I used a large stirring rod to thoroughly mix the reactants while
observing the signs of chemical reactions in each cylinder. I then let the reaction mixtures sit
undisturbed for ten minutes to allow the precipitates to settle. Once the precipitates settled
leaving a rust-colored precipitate I recorded the volume given off. I did the exact same
procedure for test two with the Copper (II) Chloride and Sodium Phosphate. I measured each
approximate volume given from data table two. From mixing both of these solutions a aquacolored precipitate was given off. I then used each of these precipitate volumes to plot versus
the mole ratio.
Data Table: 1 For Reaction of Iron(III) Nitrate with Sodium Hydroxide
Cylinder
Fe(NO3)3, 0.1 M,
mL
NaOH,0.1 M, mL
Fe:OH Mole
Ratio
Volume
Precipitate (mL)
1
5
2
10
3
12
4
15
5
17
6
20
7
24
55
1:11
50
1:5
48
1:4
45
1:3
43
2:5
40
1:2
36
2:3
4.0
6.75
7.6
11.0
8.9
5.1
2.4
Data Table: 2 For Reaction with Copper (II) Chloride with Sodium Phosphate
Cylinder
CuCl2, 0.05 M,
mL
Na3PO4, 0.05 M,
1
10
2
20
3
24
4
30
5
36
6
40
7
50
50
40
36
30
24
20
10
mL
C:PO4 Mole
Ratio
Volume
Precipitate (mL)
1:5
1:2
3:3
1:1
3:2
2:1
5:1
10.9
40.0
39.2
50.0
49.8
50.0
31.2
Conclusion:
Post-Lab Calculations and Questions:
1. On graph paper, plot the milliliters of reactant #1 versus volume of precipitate for each
reaction. For the Copper (II) Chloride graph, draw the two best-fit straight lines through
the data points and determine their point of intersection. (See graph paper)
2. For the Iron Nitrate graph, draw the best-fit line through the ascending data, and a
smooth curve through the descending data. Determine their intersection point. From
the point of intersection, determine the stoichiometric mole ration for each reaction.
Write out the correct balanced equation for each reaction. (See graph paper)
3. Explain how this method allows you to find the mole ratio of reactants. This method
allows me to find the mole ration by taking the different ratios of reactants and
measuring them to find the volume of the precipitate. From this we can plot the
volumes versus the mole ratios and find an optimum mole ratio by drawing best-fit lines
through the points and find the point of intersection.
4. Why must you keep a constant volume of reactants? It must be kept because in this
method of continuous variations, the total number of moles of reactants must be kept
constant.
5. Is it necessary that the concentrations of the two solutions be the same? Yes because in
this case they must be the same since I was trying to find the different mole ration of
the reactants.
6. What is meant by the term limiting reagent? It’s the reactant that runs out first and
limits the amounts of products that can form.
7. Which reactant is the limiting reagent along the upward sloping line of your graph?
Which is the limiting reagent along the downward sloping line? The limiting reagents
along the upward sloping lines of my graphs would be Fe(NO3)3 and CuCl2 because they
are what ran out first because there is less of them of the whole solution before the
peak of the graph. NaOH and Na3PO4 are the limiting reagents along the downward
sloping lines because there are less of them in the whole solution after the peak on the
graphs.
8. Why is it more accurate to use the point of intersection of the two lines to find the mole
ratio rather than the ratio associated with the greatest volume of precipitate? It is more
accurate to use the point of intersection of the two lines to find the mole ratio rather
than the ratio associated with the greatest volume of precipitate because in my lab I
may not have got the best mole ratio, but finding the intersecting of the lines would tell
what it would be.
Conclusion:
Through this experiment, I was able to learn how to find the optimum mole ratio for the
formation of a precipitate in a double replacement reaction and use that information to
predict the chemical formulas of the precipitate from each of the two tests I completed. I
believe both of my tests went well and I was able to find the mole rations and predict the
chemical formulas of each reaction based off my graphs and data. If my data was off it could
have been from unclean graduated cylinders, faulty chemicals, or false readings from the
volumes of the precipitates.