In these lessons, we will consider how to calculate the percent yield from a reaction
and the percent purity of the product obtained.
Lessons on Chemistry
What is Yield & Purity?
The yield is the amount of
product you obtain from a reaction. Suppose we own a factory that
makes fertilizers or paint. We will want the highest yield
possible, for the lowest cost.
If we have a factory that makes medical drugs then the yield will
still be important, but the purity
of the product may be even more important. This is because the
impurities may harm the people using the drugs.
How to calculate the percent yield?
The formula for percent yield
The medical drug aspirin is made from salicylic acid. 1 mole of
salicylic acid gives 1 mole of aspirin. Given that the chemical
formula for salicylic acid is C7H6O3
and the chemical formula for aspirin is C9H8O4.
In an experiment, 100.0 grams of salicylic acid gave 121.2 grams
of aspirin. What was the percent yield?
Step 1: Calculate
the Mr (relative molecular mass) of the substances.
: C = 12, H = 1, O = 16
: salicylic acid = 138, aspirin = 180.
Step 2: Change
the grams to moles for salicylic acid
138 g of salicylic acid = 1 mole
So, 100 g = 100 ÷ 138 mole = 0.725 moles
Step 3: Work
out the calculated mass of the aspirin.
1 mole of salicylic acid gives 1 mole of aspirin
So, 0.725 moles gives 0.725 moles of aspirin
0.725 moles of aspirin = 0.725 × 180 g = 130.5 g
So, the calculated mass of the reaction is 130.5 g
Step 4: Calculate
the percent yield.
The actual mass obtained is 121.2 g
So, the percent yield = 121.2 ÷ 130.5 × 100% = 92.9%
How to calculate the percent yield of a chemical reaction?
Consider a 3.52-g sample of CaCO3
(99.87% pure) in a
flask and a 100.0 mL sample of vinegar (5% acidity) in a graduated
cylinder. The combined mass of both reagents and containers is
255.98 g. After swirling the reaction mixture for about twenty
minutes, the combined mass of the reaction mixture and containers
is found to be 254.46 g. What is the percent yield of carbon
dioxide in this experiment?
How to solve Theoretical, Actual and Percent Yield Problems?
How to find percent yield for chemical experiments?
This chemistry tutorial cover the difference between actual,
theoretical and percent yields and include examples of how to
calculate theoretical and percent yields.
The theoretical yield
is the maximum amount of product that can be obtained in a chemical reaction. It is calculated from the limiting agent.
The actual yield
is the amount of product actually obtained from a chemical reaction. It must be experimentally determined and is never more than the theoretical yield.
Example: If 16.0 g of CaCO3
are decomposed and 7.54 g of CaO are obtained, what is the percent yield of CaO obtained?
Step 1: Always check to make sure the equation is balanced.
Step 2: Calculate the theoretical yield of CaO.
How to calculate the limiting reactant and the percent yield?
The concepts of limiting reagent, theoretical yield, and percent
yield are discussed. A sample problem that resembles a typical
test question is included.
Example: 32g of O2
reacts with 11g of C3
a) What is the limiting reagent?
b) What is the theoretical yield of H2
c) Experiment gives 10g H2
O, what is the percent yield?
This video explains the concept of a limiting reactant (or a
limiting reagent) in a chemical reaction and how to calculate the limiting reactant and the percent yield in a chemical reaction?
How to find the 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 be impure.
The percent purity of a sample describes what proportion of that
sample, by mass, is composed of a specific compound or element.
The formula for percent purity
The aspirin from the above experiment was not pure. 121.2 g of
solid was obtained, but analysis showed that only 109.2g of it was
aspirin. Calculate the percent purity of the product.
Percent purity = 109.2 ÷ 121.2 × 100% = 90.0%
Chalk is almost pure calcium carbonate. We can work out its
purity by measuring how much carbon dioxide is given off. 10 g of
chalk was reacted with an excess of dilute hydrochloric acid.
2.128 liters of carbon dioxide gas was collected at standard
temperature and pressure (STP).
The equation for the reaction is
CaCO3 (s) + 2HCl (aq) → CaCl2
(aq) + H2O (l) + CO2 (g)
Calculate the Mr of calcium carbonate
: Ca = 40, C = 12, O = 16)
Step 2: Calculate
the grams from the volume
1 mole of CaCO3
gives 1 mole of CO2
1 mole of gas has a volume of 22.4 liters at STP.
22.4 liters of gas of gas is produced by 100 g of calcium
and 2.128 liters is produced by 2.128 ÷ 22.4 × 100 = 9.5 g
Step 3: Calculate
the percent purity
There is 9.5 g of calcium carbonate in the 10 g of chalk.
Percent purity = 9.5 ÷ 10 × 100% = 95%
How to calculate the percent purity of the original sample?
Example: We have 13.9 g sample of impure iron pyrite. The sample is heated
to produce iron(III) oxide and sulfur dioxide. If we obtained 8.02
g sample of iron(III) oxide, what was the percentage of iron
pyrite in the original sample?
How to find the Percentage Purity?
Example: When dilute hydrochloric acid is added to 5.73 g of contaminated
calcium carbonate 2.49g of carbon dioxide is obtained. Find the
percentage purity of the calcium carbonate.
How to find the Percentage Purity (harder question)?
Example: 1 g of marble chip was dissolved in 25 ml of 1.3 mol of
hydrochloric acid. The unreacted acid was then neutralized by 14
ml of 1 mol of sodium hydroxide. Calculate the percentage, by
mass, of calcium carbonate in the marble chip.
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