Stoichiometry
The term "stoichiometry" refers to the quantitative relationships between reactants and
products in chemical reactions. In a balanced chemical equation, the coefficients (or
mole ratios) represent the stoichiometry of the reaction.
Often reactions occur with one reactant in excess of the stoichiometric amount that is
needed. The reactant not in excess is called the "limiting reagent". The limiting reagent
is completely consumed during the reaction and limits the amount of product that is
produced. Determining which reactant is the limiting reagent will allow the calculation of
the theoretical yield. The theoretical yield is the maximum amount of a product that can
be produced from the starting amounts of the reactants.
The mass of each reactant will be measured exactly before the reaction, and the mass
of one of the products will be determined at the end of the reaction. The mass of the
product collected is called the "actual yield". Due to spillage, glassware, and side
reactions, the actual (obtained) yield should be less than the theoretical yield. However
if the product is not completely dry, the actual yield may be greater than the theoretical
yield, resulting in a percent yield greater than 100 %.
Today you are working at a pharmaceutical company that manufactures Over the
Counter (OTC) medications that are used as antacids. Your boss has asked you
to run two different experiments and to determine the percent yield for each
reaction, so that the company can decide whether these procedures are feasible
in the manufacturing of these OTC medications.
In Part A of this experiment you will be reacting sodium carbonate with calcium
chloride to produce a common substance that is marketed as an antacid. Can
you predict the products of this reaction? (Try writing a balanced reaction on
your data sheet.) This common substance has several names and uses. It is also
found in Over the Counter calcium supplements to strengthen bones. What is the
name of some OTC products that contain this substance? What do geologists
call this substance when it is found in nature?
In Part B of this experiment you will be reacting magnesium sulfate heptahydrate
with sodium hydroxide to produce another OTC medicine that is also an antacid.
Can you predict the products of this reaction? (Try writing a balanced reaction
on your data sheet.) Do you know what this OTC medication is called? What is
the name of some OTC products that contain this substance?
Objectives:
1. Use the analytical balance and the tare function to determine an exact weight of
a reactant.
2. Write a balanced molecular equation.
3. Use a Buchner funnel to filter a solution and isolate the solid.
4. Determine the molar mass of a compound.
5. Determine the limiting reagent and calculate the theoretical yield of a reaction.
6. Calculate percent yield.
Prior to lab read the sections of our textbook that address the topics: stoichiometry,
molar mass, limiting reagents, theoretical yield, percent yield.
Chemicals:
Solid Na2CO3, CaCl2, NaOH, and MgSO4 . 7H2O
Acetone,
R.O. Water
Equipment:
Oven
Beakers
Buchner funnel
side-arm vacuum flask
glass stirring rods
petri dishes
vacuum tubing
Filter papers: size # 1 for calcium carbonate
size # 4 for magnesium hydroxide)
Sample Calculations:
A CHM 120 student weighs out .600 grams lead (II) nitrate Pb(NO3)2 and .500 grams
sodium iodide, NaI. Each reactant is dissolved in 50.0 ml of water, and then mixed
together.
Lead (II) iodide is produced as a solid and is filtered out of the mixture. The student
collected 0.651 grams PbI2.
The balanced reaction is written below.
Pb(NO3 )2 (aq) + 2 NaI (aq)
→
PbI2 (s)
+ 2 NaNO3 (aq)
In the balanced equation, lead nitrate and sodium iodide react in a 1:2 mole ratio to
produce 1 mole of lead iodide. The mole ratios or coefficients of the balanced equation
represent the stoichiometry of the reaction.
To determine the theoretical yield of lead iodide PbI2, the limiting reagent must be
determined. First the number of moles of lead nitrate and the number of moles of
sodium iodide must be determined, and then the number of moles of lead iodide
produced from each reagent must be calculated.
Calculate moles of lead nitrate:
Molar mass of Pb(NO3 )2 = 331.2 g / mole
# moles Pb(NO3 )2 = .600 grams x 1 mole / 331.2 grams
= .00181 moles Pb(NO3 )2
Calculate moles of sodium iodide.
Molar Mass of NaI = 149.9 g / mole
# moles NaI = .500 grams x 1 mole/ 149.9 grams
= .00334 moles NaI
Determination of the limiting reagent and the theoretical yield.
Pb(NO3 )2
+ 2 NaI
→
PbI2
+ 2 NaNO3
The mole ratio between Pb(NO3 )2 and PbI2 is 1:1 .
If .00181 moles of Pb(NO3 )2 reacts → .00181 moles PbI2 forms
The mole ratio between NaI and PbI2 is 2:1.
If .00334 moles NaI reacts →
.00167 moles PbI2 forms.
The number of moles of sodium iodide limits the amount of PbI2 that is produced.
Only .00167 moles PbI2 forms. The limiting reagent is the sodium iodide.
NaI determines the amount of lead iodide that can be produced. The lead nitrate is in
excess, not all of it reacts. Some lead nitrate remains unreacted.
Calculate theoretical yield of lead iodide.
Molar Mass of PbI2 = 461.0 g / mole
Theoretical Yield = .00167 moles PbI2 x 461.0 grams / mole PbI2
Theoretical yield = 0.770 grams PbI2
Determination of % yield, based on actual yield.
In this experiment, the actual yield of PbI2 collected was 0.651 grams.
Percent yield = % Yield = (Actual yield / Theoretical yield) x 100
= ( .651 g / .770 g) x 100 = 84.5 %
Download data spreadsheets
Part A: Reaction of sodium carbonate and calcium chloride:
Using chemical formulas, write a balanced equation for this reaction at the top of your
data page. Include phases.
sodium carbonate + calcium chloride --> calcium carbonate + sodium chloride
1. Weigh out approximately 1.000 grams each of sodium carbonate and calcium
chloride.
a. Use the analytical balance, the tare function and a plastic weigh tray.
b. Record the EXACT masses (+- 0.001g) in the data table.
2. Add 25 ml of R.O. water to two clean 50 mL beakers.
a. Add the sodium carbonate to one beaker and dissolve the solid by stirring
with a stirring rod.
b. Add the calcium chloride to the other beaker and dissolve the solid by stirring
with a stirring rod.
3. Mix the solutions containing the dissolved sodium carbonate and calcium
chloride. A cloudy white suspension will form.
Filtering and collecting the product:
1. Obtain a piece of # 1 filter paper. RECORD the mass of the filter paper and a
petri dish. (# 1 filter paper is used for filtering crystalline precipitates such as
calcium carbonate.)
2. Filter the solid from the suspension.
a. Attach a Buchner funnel to a vacuum flask. Use a ring clamp to secure it so
that it does not tip over.
b. Add the weighed piece of filter paper to the septum of the funnel.
c. "Wet" the filter paper with a small amount of water. The paper should be
secure in the filter, with no holes showing.
d. Attach a vacuum hose and turn on the vacuum slowly.
e. Pour the suspension slowly in the center of the filter paper. Increase the
vacuum.
f. Rinse any remaining solid from the beaker with some water.
3. Rinsing and collecting the calcium carbonate.
a. Rinse the crystals with 10 ml of acetone to remove any water.
b. Repeat the acetone rinse with another 10 ml.
c. Allow the crystals to dry with the vacuum on for 15 minutes.
d. Turn off the vacuum and disconnect the vacuum hose from the filter.
e. Using a flat spatula, carefully lift the filter paper and crystals. Place the filter
paper and crystals in the previously weighed petri dish.
f. Dry the crystals in the oven on the filter paper in the petri dish for 30 minutes,
or longer until the paper and the crystals are completely dry.
g. While the product is filtering and drying, you can calculate the limiting reagent
and the theoretical yield of the product on your data sheet. Show your
calculations.
h. Allow the dish and crystals to cool for 5 minutes and then weigh it.
4. Dispose of the solid calcium carbonate in the waste basket.
Calculate the % yield of the product using the actual yield and the theoretical yield.
Record the % yield on your data sheet.
Part B: Reaction of NaOH and magnesium sulfate heptahydrate:
Safety Precautions: NaOH can be very corrosive and can cause severe burns.
Do not touch solid NaOH! Keep safety glasses on at all times to prevent eye
injuries.
1. Using chemical formulas, write a balanced reaction for this experiment at the top
of your data page. Include phases.
sodium hydroxide + magnesium sulfate heptahydrate -->
magnesium hydroxide + sodium sulfate + water
2. Weigh out approx. 1.500 grams sodium hydroxide and 5.500 grams magnesium
sulfate heptahydrate.
a. Use the analytical balance, the tare function and a plastic weigh tray.
b. Record the EXACT weights (+- 0.001g) in the data table.
3. Add 25 ml of R. O. water to two clean 50 mL or 100 mL beakers..
a. Add the sodium hydroxide to one beaker and dissolve the solid by stirring
with a stirring rod.
b. Add the magnesium sulfate heptahydrate to the other beaker and dissolve
the solid by stirring with a stirring rod.
4. Mix the solutions containing the sodium hydroxide and magnesium sulfate
heptahydrate. A cloudy white suspension will form.
Filtering and collecting the product:
1. Obtain a piece of # 4 filter paper. RECORD the mass of the filter paper and petri
dish to be used. (# 4 filter paper is used for filtering gel and coarse precipitates:
magnesium hydroxide.) This suspension will filter slowly.
2. Filter the solid from the suspension.
a. Attach a Buchner funnel to a vacuum flask. Secure it with a ring clamp.
b. Add the weighed filter paper to the septum of the funnel.
c. "Wet" the filter paper with a small amount of water. The paper should be
secure to the septum, with no holes showing.
d. Attach a vacuum hose and turn on the vacuum slowly.
e. Pour the suspension in the center of the filter paper. Increase the vacuum.
f. Rinse any solid from the beaker with some water.
3. Rinsing and collecting magnesium hydroxide.
a. Rinse the crystals with 10 ml of acetone to remove any water.
b. Repeat the acetone rinse with another 10 mL.
c. Allow the crystals to dry with the vacuum on for 15 minutes.
d. Using a flat spatula, carefully lift the filter paper and crystals. Place the
filter paper and crystals in a previously weighed petri dish.
e. Dry the crystals in the oven on the filter paper in a petri dish for 30
minutes, or longer until the paper and the crystals are completely dry.
f. While the product is filtering and drying, you can calculate the limiting
reagent and the theoretical yield of the product on your data sheet. Show
your calculations.
g. Allow the dish and crystals to cool for 5 minutes and then weigh it.
4. Dispose of the solid magnesium hydroxide in the waste basket.
5. Clean all glassware and return all equipment to its correct location.
Calculate the % yield of the product using the actual yield and theoretical yield
values. Record the % yield on your data sheet.
FOLLOW-UP QUESTIONS:
1. Pharmaceutical companies and chemical manufacturing plants try to obtain high
percent yields of pure products. List and explain several sources of error you observed
that may have affected your percent yield of product?
2. If the product was not completely dry when it was removed from the oven and
massed, how would your percent yield be affected?