Lab Final – Stoichiometry of Chemical Reactions
Background
How is the atmosphere inside a manned spacecraft controlled? What prevents a buildup of CO 2 from the respiration of the
astronauts? Chemical engineers solved this problem for the Apollo moon program and the space shuttle by using lithium hydroxide
canisters that filtered the cabin air flow system. Lithium hydroxide has a high affinity for CO 2:
CO2(g) + 2LiOH(s) → Li2CO3(s) + H2O(g)
The irreversible reaction converts CO2 into Li2CO3 and water vapor. Knowing how much CO2 each astronaut exhales every
day and the number of astronauts for a mission of given days, chemical engineers can calculate how many canisters are needed to
absorb the CO2 produced. This calculation was done by using the stoichiometry of the above balanced equation.
Stoichiometry enables chemist to calculate the quantity of reactants needed to make a given amount of a product. The
information in a balanced equation indicates the proportions of reactants required to produce the products. Mole ratios derived
from the coefficients are used to determine moles of reactants and products and to calculate the mass of the reactants and products
involved. The amount of products may be limited by the amount of one of the reactants, termed the limiting reagent. The reactant
that is not completely used is termed the excess reagent.
The Job’s plot, also known as the method of continuous variation, can be used to determine the molar ratio of reactants
and that maximizes the amount of the product. This method employs a series of prepared solutions, each containing the same total
moles of reactants but in different molar ratios. The maximum amount of product is produced when the molar ratio is closest to the
stoichiometric molar ratio of the balanced equation for the reaction. Quantitative measurements of the reaction may include change
in temperature, color intensity, mass of a precipitate, or the volume of a gas. Measurement data is then graphed against the
reactant ratio to determine the correct stoichiometric molar ratios for the reactants. The following example illustrates the use of a
Job’s plot to determine correct molar ratios for a balanced equation.
Using a Job’s plot, find the proper molar ratios (coefficients) for the reaction of 0.5M solutions of reactant A and reactant B.
Product C is a precipitate that is weighed for each trial.
____A + ____B → C
The following data were collected for six trials:
Trial Number Volume of A (ml) Volume of B (ml) Grams of C
1
5.0
0.0
0.00
2
4.0
1.0
0.33
3
3.0
2.0
0.67
4
2.0
3.0
1.00
5
1.0
4.0
0.50
6
0.0
5.0
0.00
The best-fit lines intersect at a volume of 2.0ml for reactant A, which produces the maximum amount of precipitate as
1.00g of product C. Since the maximum volume of both reactants A and B is kept at 5.0ml, the volume of B needed to react with A
must be 3.0ml. The ideal stoichiometric ratio, then, of A to B to produce the maximum grams of C must be 2:3 moles.
2A + 3B → C
Due to the mass-mole relationship, if the molar concentration and volume of a solution are known, then the mass of the
𝑚𝑜𝑙𝑒𝑠 𝑠𝑜𝑙𝑢𝑡𝑒
solute can be calculated.
𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦 =
𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
Step 1: Calculate the moles of solute.
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒
Step 2: Convert moles of solute to mass of solute.
𝑚𝑜𝑙 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒 ×
= 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒
𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒
And now… the All-Important, Actual Lab Experiment!
In this experiment, the total volume of reactants is constant; however, the volume of each reactant varies from one mixture to
another. The concentration is known for each solution of the reactants. The change in temp for the reaction is measured as the volume
of each reactant is systematically varied to determine the best molar ratio of reactants.
The change in temperature (∆T) is the difference between the final temperature (Tf ) and the initial temperature (Ti ) of the
acid and base. If the temperatures of the acid and base are not the same initially, an average of the two different temperatures must
be calculated to determine the initial temperature at mixing. For example,
if 10ml of base at 30°C and 40ml of acid at 25°C were mixed, here’s how you find the initial temp for the reaction:
(10𝑚𝑙 × 30𝑜 𝐶) + (40𝑚𝑙 × 25𝑜 𝐶)
= 26𝑜 𝐶
50𝑚𝑙
Pre-laboratory Questions
1. What observations can be made that indicate a chemical reaction has occurred?
2. Calculate the mass of calcium chloride that is in a 5ml sample of a 2M solution.
3. Determine the limiting reagent if 6ml of 1M sodium hydroxide is added to 4ml of 1M copper sulfate.
2NaOH(aq) + CuSO4(aq) → Na2SO4(aq) + Cu(OH)2(s)
4. If the initial temperatures of the base and the acid are not the same, then you must make a correction for the temp difference.
Calculate the correct change in temp for the following sample data. The initial temp of 30ml of base is 30°C, and the initial temp
of 20ml of acid is 35°C. The final temperature after mixing these two solutions is 42°C. Use the background to aid you.
Procedure
Wear safety goggles in this lab. Wash your hands thoroughly with soap and water before leaving the lab.
1. Your teacher will assign your group an unknown acid, identified as either A or B, to use in your experiment.
2. Label your 250ml beakers “1M sodium hydroxide” and “1M Unknown Acid A or B,” as appropriate. In your beakers, collect the
following from the supply station: 180ml of 1M sodium hydroxide
180ml of 1M Unknown Acid (A or B, as assigned)
3. Measure the temp of the 1M sodium hydroxide and the temp of the unknown acid. Record the data in the data table.
If the solutions are not the same temp, make a correction for the temp difference as discussed in the background.
5. Rinse the thermometer with distilled water, and thoroughly dry it.
6. Create a reaction chamber by nesting one foam cup inside the other then placing them in a 400ml beaker.
Place the lid on the inner foam cup and insert the thermometer through the slot in the lid.
7. Remove the reaction chamber lid. With the 100ml cylinder, measure 40ml of 1M sodium hydroxide and pour it into the foam cup.
8. With the 10ml cylinder, measure 10ml of the unknown acid and add it to the sodium hydroxide in the cup.
Quickly replace the lid and the thermometer.
9. Carefully stir the mixture by moving the 400ml beaker in a gentle circular motion on the lab table. Observe the
max temp reached as the reaction occurs. Record this max temp as the final temp of the mixture in the data table.
10. Pour the mixture in the 1000ml waste beaker.
11. Rinse the foam cup, thermometer, and graduated cylinder with distilled water and dry them.
12. Repeat steps 7-11 for the remaining reactions. Record the max temp in the Data Table.
Since the first and last trials do not require any mixing of chemicals, record ∆T as 0.
13. Calculate and record the change in temperature in for each acid-base reaction in the appropriate data table.
14. Complete the analysis section before the end of the period. A copy of the data analysis is due by the end of the period.
Data Table
Analysis Table
ml of 1M NaOH ml of 1M acid Final Temp Initial Temp ∆T (oC)
Reactants
NaOH
Acid ____
50.0
0.0
Volume
40.0
10.0
Mass of
30.0
20.0
Moles of
20.0
30.0
10.0
40.0
0.0
50.0
Questions
5. On the basis of the graph, determine
the molar ratio for the reaction of NaOH
and your unknown acid
Molar ratio of NaOH to acid: _______
6. Why is it more reliable to use the point
of intersection of the two best fit lines to
determine the molar ratio rather
than use the molar ratio associated
with the greatest temperature change?
7. Write the balanced chemical eqns for
the following reactions:
sodium hydroxide with hydrochloric acid,
sodium hydroxide with phosphoric acid,
sodium hydroxide with sulfuric acid.
1. Graph the volume of NaOH used versus change in temp for your unknown acid.
Write the molar ratio of hydroxide
to acid for each equation.
2. Draw a best fit line through the ascending points, and a second best fit
8. Identify your acid as one of these acids.
line through the descending points. The two lines should intersect. At the
9. Applying the molarity formula to your
intersection, what is the volume of sodium hydroxide and unknown acid?
experimental data, determine the number
Volume of NaOH = ________ml Volume of Acid = ________ml
of moles and mass of reactants for each
3. Name the limiting reagent along the ascending line on the graph.
reaction. Record in the analysis table.
4. Name the limiting reagent along the descending line on the graph.
Lab Final – Stoichiometry of Chemical Reactions
Name ____________________
Objectives:
Procedure:
Pre-laboratory Questions
1. What observations can be made that indicate a chemical reaction has occurred?
2. Calculate the mass of calcium chloride that is in a 5ml sample of a 2M solution.
3. Determine the limiting reagent if 6ml of 1M sodium hydroxide is added to 4ml of 1M copper sulfate.
2NaOH(aq) + CuSO4(aq) → Na2SO4(aq) + Cu(OH)2(s)
4. If the initial temps of the base and the acid are not the same, then you must make a correction for the temp difference.
Calculate the correct change in temp for the following sample data. The initial temp of 30ml of base is 30°C,
and the initial temp of 20ml of acid is 35°C. The final temperature after mixing these two solutions is 42°C.
Data Table
ml of 1M NaOH ml of 1M acid
50.0
0.0
40.0
10.0
30.0
20.0
20.0
30.0
10.0
40.0
0.0
50.0
Questions
Final Temp
Initial Temp
∆T (oC)
Analysis Table
Reactants
Volume
Mass of
Moles of
NaOH
Acid ____
Volume of Acid = ________ml
3. Limiting reagent along
the ascending line
on the graph. ____________
4. Limiting reagent along
the descending line
on the graph. ____________
5. Molar ratio of NaOH
to acid:
____________
6.
8. Your Acid: __________
7. Balanced equations:
2. Volume of NaOH = ________ml
9. Show work here:
Lab Final – Stoichiometry of Chemical Reactions
Data Table
ml of 1M NaOH ml of 1M acid
50.0
0.0
40.0
10.0
30.0
20.0
20.0
30.0
10.0
40.0
0.0
50.0
Questions
Final Temp
2. Volume of NaOH = ________ml
Volume of Acid = ________ml
5. Molar ratio of NaOH to acid:
7. Balanced equations:
9. Show work here:
Initial Temp
∆T (oC)
Name ___________________
___________________
___________________
___________________
Analysis Table
Reactants
Volume
Mass of
Moles of
3. Limiting reagent along
the ascending line
on the graph. ____________
NaOH
Acid ____
4. Limiting reagent along
the descending line
on the graph.
____________
____________
8. Your Acid: __________