In this lesson, we will consider how to calculate the percent yield from a reaction and the percent purity of the product obtained.
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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.
The formula for percent yield is:
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.
Ar : C = 12, H = 1, O = 16
So, Mr : 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%
The following video shows another example of calculating percent
Example: 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?
The concepts of limiting reagent, theoretical yield, and percent
yield are discussed. A sample problem that resembles a typical
test question is included.
This video explains the concept of a limiting reactant (or a limiting reagent) in a chemical reaction. He also shows you how to calculate the limiting reactant and the percent yield in a chemical reaction.
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 is:
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)
Step 1: Calculate the Mr of calcium carbonate
Ar: Ca = 40, C = 12, O = 16)
Mr of CaCO3 = 100
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 carbonate
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%
Percent purity lesson
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?
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