Homework: p. 262 #31-33, p. 264 # 34-37, p. 270 #1-3
Theoretical Yield = The maximum amount of product that cold be formed from given reactants.
--Theoretical yield is the answer from the stoichiometry calculation = maximum possible amount assuming
reactions goes to 100% completion
---Number based upon the “theoretical” mathematical equation – no experiments were conducted.
Actual Yield = The amount of product that actually forms when the reaction is carried out in the lab.
--Due to a variety of factors, the actual amount will ALWAYS be less than the theoretical yield.
---In real life, we never get as much as we anticipate, this “actual” amount from the lab experiment.
Percent Yield = The ratio of the actual yield to the theoretical yield expressed as a percentage.
Percent Yield =
Actual ,
Theoretical
x 100
Percent Purity= When we make something in a chemical reaction, and separate it from the final mixture, it will
still have small amounts of other substances mixed with it. It will impure.
Percent Purity= mass of pure product x 100
Mass of impure product
EXAMPLE) How many grams of hydrofluoric acid (HF) are required to react completely with 23.68 grams of
calcium hydroxide in the following reaction?
_1_ Ca(OH)2 + __2_ HF  __1_ CaF2 + __2_H2O
1 mol Ca(OH)2
2 mol HF
20.01 g HF
23.68 g Ca(OH)2 x ------------------ x ----------------- x ------------------ = 12.79 g HF
74.10 g Ca(OH)2
1 mol Ca(OH)2
1 mol HF
Based on the stoichiometry of this problem, the Theoretical Yield for this problem is 12.79 g HF. But, when this
experiment was conducted, an Actual Yield of only 10.41 g HF was collected. Using the equation for percent
yield, the Percent Yield of this experiment was 81.39%.
10.41g HF , x 100 = 81.39%
Actual , x 100 =
Percent Yield =
12.79g HF
Theoretical
FACTORS THAT AFFECT YIELD
1) Competing reactions
2) Reaction rates
3) Purity of the reactants
4) Lab techniques used
EXPLANATION
EXAMPLE
A reaction that occurs along with
the principal reaction and that
involves the reactants and/or
products of the principal reaction.
How fast the reaction occurs (can
be affected by temperature,
pressure, surface area, reaction
vessel conditions).
2P + 3Cl2  2PCl3
PCl3 + Cl2  PCl5 (Cl2 and
PCl3 are being consumed in
this side reaction).
A slow reaction does not go to
completion and reaction
products are collected too soon.
If you assume your reactants are
pure when they are not, your actual
yield will be much less than your
calculated theoretical yield.
Reactants may not be pure
(contaminated with other
chemicals).
How you carry out the laboratory
(techniques and equipment used).
Impure reactants contain
contaminants. Incorrect
theoretical yield calculations
are based on impure reactants
and the mass of the impurity is
not considered.
Improper lab techniques reduce
actual product yield. Some of
the product may cling to the
reaction vessel, stirring rods,
spatulas, or other equipment.
STEPS TO DETERMINING PERCENTAGE YIELD:
Step 1. You must have a balanced chemical equation for the reaction.
Step 2. From the weights of the reactants and reagents, calculate the moles of each.
Step 3. Determine which of the reactants or reagents is the limiting reagent.
Step 4. From the moles of limiting reagent and the balanced equation, calculate the number of moles of
product that could theoretically be produced.
Step 5. From the theoretical moles of product, calculate the theoretical yield in grams.
Step 6. Percent yield = [actual yield (grams) / theoretical yield (grams)] * 100%
Percent Yield
Most reactions do not go to completion, and so the actual yield is less than the percent yield. The
percent yield is calculated as
 actual yield 
Percent yield = 
 100%
theoretical yield 
___________________________________________________________________________
Exercises: Use your own paper if you want more space.
1. Ammonia is produced by the reaction
3 H2(g) + N2(g)  2 NH3(g)
(a) If N2(g) is present in excess and 55.6 g of H2(g) reacts, what is the theoretical yield of NH3(g)?
(b) What is the percent yield if the actual yield of the reaction is 159 g of NH3(g)?
Answers: #1(a) 313 g NH3(g); (b) 50.8% yield.
___________________________________________________________________________
2. Methyl alcohol (wood alcohol), CH3OH, is produced via the reaction
CO(g) + 2 H2(g)  CH3OH (l)
A mixture of 1.20 g H2(g) and 7.45 g CO(g) are allowed to react.
(a) Which reagent is the limiting reagent?
(b) What is the yield of CH3OH? [Assume theoretical yield in g is what is wanted here.]
(c) How much of the reagent present in excess is left over?
(d) Suppose the actual yield is 7.52 g of CH3OH. What is the % yield?
Answers: # 2(a) CO is the limiting reagent; (b) 8.52 g CH3OH; (c) 0.13 g H2; (d) 88.3%\
3. Examine the following reaction: ____ Mg + ____ HNO3  ____ Mg(NO3)2 + ____ H2
(a) If I start this reaction with 40 grams of magnesium and an excess of nitric acid, how many grams of
hydrogen gas will I produce?
(b) If 1.7 grams of hydrogen is actually produced, what was my percent yield of hydrogen?
Answers: # 3 (a) 3.3 g H2 (b) 52 %
___________________________________________________________________________
4. Examine the following reaction: ____ NaHCO3  ____ NaOH + ____ CO2
(a)
If 25 grams of carbon dioxide gas is produced in this reaction, how many grams of sodium
hydroxide should be produced?
(b)
If 50 grams of sodium hydroxide are actually produced, what was my percent yield?
Answers: # 4 (a) 22.7 g NaOH (b) 220 %
___________________________________________________________________________
5. If a 156.0 g of Cu ore contains 60.00 g of pure Cu metal. What is the percent purity?
Answers: #5. 38.46 % purity
___________________________________________________________________________
6.
There are 6.5 grams of impure Zn, the percent purity is 87%, what is the mass of the pure Zn?
Answers: # 6. 5.6 g of pure Zn
___________________________________________________________________________