Percent Yield & Percent Purity

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.

In this lesson, we will consider how to calculate the percent yield from a reaction and the percent purity of the product obtained.

Finding the percent yield

The formula for percent yield is:

Example:

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?

Solution:

Step 1: Calculate the M_r (relative molecular mass) of the substances.

A_r : C = 12, H = 1, O = 16

So, M_r : 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 yield.

Finding 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.

The formula for percent purity is:

Example:

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.

Solution:

Percent purity = 109.2 ÷ 121.2 × 100% = 90.0%

Example:

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)

Solution:

Step 1: Calculate the M_r of calcium carbonate

A_r: Ca = 40, C = 12, O = 16)

M_r 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%

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