Stoichiometry: Synthesis of
Garden Lime
Standard: The student will explore the mathematics of chemical formulas and
equations.
Have you ever wondered why hot dogs are sold in packages of
10, but hotdog buns are sold in packages of 8?
T
his is an example of a ratio. Stoichiometry is the scientific word for ratios of moles. It is
the study of the quantitative relationships between reactants and products in a chemical
reaction. These relationships are often referred to as mole-to-mole ratios. Mole-to-mole
ratios help scientists know how much of each reactant is needed to produce a sufficient
amount of product. In this lab, you will observe the importance of mole-to-mole ratios when
making the main component in lime that is put on a garden!
Introduction
The manufacturers of hotdogs and hotdog buns would not make very good chemists. Hotdogs
are sold in packages of 10 where hotdog buns are sold in packages of only 8. If one package of each is
purchased, how many hotdog-on-buns can you make? The answer of course is 8. This means there are 2
hotdogs left over. Now examine the equation below and try to fix their mistake. How many packages of
hotdogs and hotdog buns must you purchase in order to not waste any hotdogs? How many hotdog-onbuns does this give you?
___10 Hotdogs + ___ 8 Hotdog buns  ___ Hotdog-on-buns
If you buy 4 packages of hotdogs and 5 packages of buns you
hotdog-on-buns.
would get 40
Balancing a chemical equation is done the same way. To
balance a chemical
equation you place numbers, called coefficients, in front of each type of molecule. Just like you cannot
change how many hot dogs there are in a package of hotdogs, you cannot change how many of each type
of atom there is in a chemical substance. You can however change the ratio of the chemical substances so
that the same number of each type of atom is on both sides of the reaction equation. In the hotdog
example there are 40 hotdogs and 40 hotdog buns on both sides of the arrow.
But what if instead you buy only 2 packages of hotdogs, and 3 packages of hotdog buns,
which would you run out of first? You would have a total of 20 hotdogs and 24 hotdog buns so you
would run out of the hotdogs first. This means you could only make 20 of the hotdog-on-buns final
product. The number of hotdogs limits the number of hotdog-on-buns you can make.
This is similar to what in chemistry is called the limiting reagent. By using stoichiometry it
is possible to calculate the limiting reagent of a reaction. Knowing which reactant will be used up
first allows chemist to calculate how much product can theoretically be made. This is often
compared to the amount that is actually made through calculating what is called the percent yield.
The percent yield is found by using the following equation:
Percent Yield = __Actual yield__ x 100
Theoretical yield
Using the hotdog example, the percent yield would be like only making 19 hotdogs when
there were enough hotdogs and buns to make 20 hotdog-on-buns. In this example the percent yield
would be calculated as follows:
Percent Yield = __19 Actual Hotdog-on-buns___ x 100
20 Theoretical Hotdog-on-buns
Percent Yield = 95%
Chemist will often try making a desired product many different ways. They will then
compare the percent yields to determine which of the methods tried gives the greatest yield.
In this laboratory exercise you will explore the concepts of limiting reagents and percent
yields through making calcium carbonate, the main component in lime. Lime is used in many
different industries, including farming. Farmers spread it on soil to adjust its pH so that their crops
will be more productive.
Calcium carbonate, CaCO3, can be made by mixing CaCl2 and K2CO3 in water. The trick is
to be sure to use the correct amount of each. For this, stoichiometry is used. Look at the following
reaction and balance it to determine the mole-to-mole ratio that would be necessary to have the
same number of each kind of atom on both sides of the equation.
___ CaCl2(aq) + ____ K2CO3(aq)  ____ KCl(aq) + ____ CaCO3(s)
When this chemical equation is balanced there is one mole of calcium chloride for every 1 mole of
potassium carbonate and together these quantities make 2 moles of potassium chloride and one mole of
calcium carbonate. The potassium chloride is soluble in water so it remains in solution. The calcium
carbonate will precipitate out as a white solid.
CaCl2(aq) + K2CO3(aq)  2KCl(aq) + CaCO3(s)
Objectives

Demonstrate the use of stoichiometry to synthesize calcium carbonate.

Practice using a balance and proper lab techniques.

Find the limiting reagent, the theoretical yield, and the percent yield.
Date:________ Name:_______________________ Period:_________
Pre-lab Questions
1. What is a limiting reagent?
_____________________________________________________________
_____________________________________________________________
1. A student used 7.15 g of CaCl2 and 9.25 g of K2CO3 to make CaCO3. The actual yield was 6.15 g of
CaCO3. Calculate the limiting reagent and the percent yield.
Procedure
Weigh into a 250 mL beaker the amount of calcium chloride (CaCl2) specified for your
group in the chart below. Record the exact mass you weigh out in the data section.
1.
2.
Measure 50 mL of distilled water into a
100 mL graduated cylinder. Pour the
water into the 250 mL beaker with the
calcium chloride.
Group A
~1.00 g CaCl2
Group B
~2.00 g CaCl2
Group C
~3.00 g CaCl2
3.
Stir the solution with a stirring rod until all of the calcium chloride is dissolved.
4.
Weigh out ~2.50 g of potassium carbonate (K2CO3) in a 50 mL beaker.
exact mass in the data section.
5.
Measure 25 mL of distilled water into a 100 mL graduated cylinder. Add the water into
the 50 mL beaker containing the potassium carbonate.
6.
Stir the potassium carbonate in the distilled water with a stirring rod until it is all
dissolved.
7.
Pour the K2CO3 solution into the 250 mL beaker that has the CaCl2 solution. Rinse the
beaker that contained the K2CO3 with a few mL of water and add this to the CaCl2
solution. Stir the mixture.
8.
As soon as the reaction begins, record your observations in the data section. Continue
stirring until you see no more precipitate forming.
9.
Set up the filtering apparatus as shown in Figure 9.1. Secure the Erlenmeyer flask to a
ring stand. Rest the funnel in it. HINT: Do NOT begin filtering yet!
Record the
10. Zero the balance and weigh a piece of medium flow filter paper and
a watch glass.
section.
Record the masses of both items in the data
11. Fold
the filter paper into quarters. Open the filter paper so that a
funnel is formed. HINT: Three sides will be together and the other
side will open out.
12. Place
the paper funnel into the glass funnel and hold it in place
with your fingers. Pour a small amount of distilled water onto the
filter paper to secure it, so that you do not have to hold it in place.
13. Filter the mixture by pouring it into the filter paper in the funnel.
Use the stirring rod and distilled water in a wash bottle to transfer the entire solid into
the filter paper. HINT: For best results, be sure to transfer all of the precipitate into the
filter paper. Use a rubber policeman if it is available to help with the transfer.
14. Rinse
the remaining solid in the filter paper twice with distilled water from a wash
bottle to rinse off excess potassium chloride (KCl). After all the liquid has filtered
through, rinse the product with approximately 5 mL of ethanol to aid in its drying.
Allow the ethanol to completely finish filtering through the paper.
15. Remove the filter paper carefully so as to not lose any product. Gently unfold the filter
paper and lay it flat on the pre-weighed watch glass to dry. HINT: Do not throw your
product away after you weigh it!
16. Allow the product to air dry until the next laboratory period.
17. In
the next laboratory period, weigh the dry product on the filter paper and watch
glass. Record the total mass in the data section. Calculate the mass of the product.
18. Complete
the data and calculations by completing each step and comparing your
results with the other groups.
19.
Place the dried product in a designated container for later use in the Rates of
Reaction experiment. Wash any dirty glassware
Data
Record the data for your group.
Group ___________
Mass of CaCl2 _______________
Mass of K2CO3 ______________
Mass of filter paper_________________
Mass of watch glass_________________
Mass of product, filter paper, and watch glass____________________
Mass of dry product_________________
Observations for the reaction:
Table 1. Final Results for EACH group
Group
Group A
Group B
Group C
Actual Mass
of CaCl2 (g)
Actual Mass
of K2CO3 (g)
Mass of Dry
CaCO3 Obtained (g)
Calculations
1.
Determine the limiting reagent for each group. Show your calculations. (Hint: See the
example in the Introduction. )
Group A:
Group B:
Group C:
2. Calculate the theoretical yield of CaCO3 that could be produced by each group and then fill in
Table 2.
Table 2: Comparison of Theoretical and Actual Yields for CaCO3
Group
Limiting Reagent
Theoretical
Yield of CaCO3
Actual Yield of
CaCO3
Group A
Group B
Group C
3. Find the percent yield your group obtained for the CaCO3.
Post Lab Questions
1. Compare your results with those of the other two groups. How does the amount
CaCO3 they obtained compare to your results?
of grams of
_____________________________________________________________
_____________________________________________________________
2. Were the results of the other two groups as you expected? Why or why not?
_____________________________________________________________
_____________________________________________________________
3. Predict what would happen if 6.0 grams of CaCl2 were used for the reaction and the amount of
K2CO3 remained the same.
_____________________________________________________________
_____________________________________________________________
Conclusion: