Chemistry Post lab

post lab chemistry lab

CHEM 108 – Lab Experiment 7
Reaction Stoichiometry
Introduction
A balanced chemical equation both identifies the reactants and products in a chemical reaction as well
as providing quantitative information about how the reaction occurs. Specifically, the coefficients in the
balanced equation provide molar ratios indicating the mathematical relationship between the
substances in the reaction. Consider the general chemical reaction of substances A and B to produce
substances C and D:
aA + bB  cC + dD
The molar relationships represented by the lower case letters can be used to convert the amount of one
substance in the reaction into any of the other substances in the reaction. For instance, if we were
interested in the mass of product C, we can convert the mass of starting material A into the mass of
product C.
mass A  moles A  moles C  mass C
The situation shown above in which an expected amount of product is determined from the starting
amount of reactant is called the theoretical yield. It is also referred to as the theoretical mass since lab
quantities are commonly measured in mass.
While the theoretical yield of a reaction is a very useful target for a chemical reaction, most reactions do
not yield the theoretical amount. A variety of factors can cause the actual yield of the reaction to be
different from the expected/theoretical yield. The actual yield for a reaction may be higher or lower
depending on the underlying factors causing the deviation.
Chemists frequently report the extent of a reaction by calculating a percent yield. The percent yield is
simply the percent ratio of actual yield to theoretical yield:
Percent Yield 
Actual Yield
x 100%
Theoretical Yield
It is not uncommon for researchers to extensively study reactions in an effort to improve the percent
yield. In fact, the logistic and economic feasibility of producing certain pharmaceutical drugs or
performing some large industrial processes rely on having sufficiently high reaction yields.
In this lab experiment, you will be experimentally and theoretically determining the mass of silver
chloride (AgCl) produced by reaction with a known mass of sodium chloride (NaCl).
Reaction Stoichiometry
© Colorado State University Department of Chemistry
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Pre‐lab Questions (10 pts) ‐ must be typed and submitted on Canvas after taking safety quiz
1. There are safety hazards in this lab experiment. Specifically, what are the safety hazards associated
with the experimental procedure? Be sure to clearly identify and explain why these are hazards.
2. Identify the waste produced in this lab and explain how and where it should be properly discarded.
3. Explain why it is not necessary to precisely measure the amount of water added to the sodium
chloride in the procedure.
4. If one mole of CO2 is produced in the following reaction, how many moles of H2O are also produced?
Show your work/explain. C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g)
5. If 3 moles of NaCl are needed to produce 1 mole of AlCl3, how many grams of NaCl are needed to
produce 200 grams of AlCl3? Show all of your work/calculations.
Experimental Procedure
Use caution when working around the Bunsen burners. DO NOT place hot objects on the countertop
or table pads since they are NOT heat resistant and this will cause them to melt and/or burn.
1. Carefully weigh out about 0.1 g of solid NaCl into a clean 250 mL beaker. Record the exact mass you
measured on your report form.
2. Add approximately 25 mL of distilled water to the NaCl in the beaker and
gently swirl to completely dissolve the solid. Note: 25 mL can be estimated
using the markings labeled on the beaker, it does not need to be precise.
3. Using a graduated cylinder, add 20 mL of 0.1 M AgNO3 solution to the
sodium chloride solution. Record careful observations of the resulting
mixture.
4. On the Bunsen burner, heat the mixture to a rolling boil and allow it to boil
for about 5 minutes until most of the cloudiness has disappeared. Take
observations throughout the boiling process and note any changes and the
time at which they take place. Note: the solution may not become totally
clear but it should improve from the original milky/cloudy appearance.
5. Leave the beaker on the stand to cool for 5‐10 minutes. Remember ‐ do
not place any hot objects onto the countertop or table pads. Also, be sure you have recorded
observations of the final mixture after boiling before you leave it to cool.
6. While waiting, weigh your empty evaporating dish and record the mass on your report form. Use a
marker and place your initials or a similar identifying mark on the evaporating dish.
7. After the solution has cooled, carefully filter the solid precipitate using gravity filtration. Pour the
liquid filtrate that you recover into the liquid waste container.
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Chem 108 Laboratory Manual
© 2019 Colorado State University Department of Chemistry
8. Using a spatula, carefully transfer and spread the precipitate out in the evaporating dish (do NOT
place filter paper in the oven). Record observations of the recovered wet solid.
9. Put the evaporating dish in the oven and let the precipitate dry for at least 30 minutes.
10. While the solid is drying in the oven, answer the questions on your lab report or work on other
assignments.
11. After 30 minutes, remove the dish from the oven. Place the dish on your ring stand to cool for 5‐10
minutes. Remember ‐ do not place any hot objects onto the countertop or table pads.
12. Weigh the dish and dried solid and record the mass on your report form. Record observations of the
final dried solid.
13. Place the solid in the chemical waste container. DO NOT RINSE ANY LIQUID INTO THE SOLID WASTE
CONTAINER. Clean your evaporating dish with a brush, soap, and hot water.
Reaction Stoichiometry
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Chem 108 Laboratory Manual
© 2019 Colorado State University Department of Chemistry
CHEM 108 – Lab Experiment 7: Reaction Stoichiometry
Name:
Section:
In‐Class Activities (20 pts)
Data:
Mass of solid NaCl
g
Mass of empty evaporating dish
g
Mass of evaporating dish and dried AgCl after heating in oven
g
Observations of the mixture after combining NaCl and AgNO3.
Observations throughout boiling process:
Observations at the end of boiling:
Observations of the recovered wet solid:
Observations of the final dried solid:
Reaction Stoichiometry
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Chem 108 Laboratory Manual
© 2019 Colorado State University Department of Chemistry
Post‐lab Assignment (36 pts)
1. Answer the following questions based on the reaction conducted in this lab experiment.
a. (4 pts) Write a balanced chemical equation for the reaction conducted in this lab. Be sure to
include all phase labels and properly balance the equation.
b. (6 pts) Calculate the theoretical mass of AgCl produced in this reaction based on the mass of
NaCl you measured at the beginning of your experiment.
c. (2 pts) Using your data, calculate the actual mass of AgCl produced in your experiment.
d. (4 pts) Calculate the percent yield of AgCl for your experiment.
2. (6 pts) Give three possible sources of error in this experiment that would lead to a percent yield
other than 100%. Clearly indicate whether each error would result in a higher or lower value and
explain why that would be the result.
3. Consider a different reaction from the one conducted in lab. When nitrogen gas (N2) reacts with
oxygen gas (O2) the result is the formation of dinitrogen trioxide gas (N2O3). At the start of a
particular experiment, 3.20 grams of O2 is reacted with excess N2. At the end of the experiment, the
amount of N2O3 actually produced by the reaction is 4.50 grams.
a. (4 pts) Write a balanced chemical equation for this reaction. Be sure to include all phase labels
and properly balance the equation.
b. (6 pts) Calculate the theoretical mass of the N2O3 in the experiment.
c. (4 pts) Calculate the percent yield of the product made in the experiment.
Reaction Stoichiometry
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Chem 108 Laboratory Manual
© 2019 Colorado State University Department of Chemistry