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

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:

*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 C_{7}H_{6}O_{3}
and the chemical formula for aspirin is C_{9}H_{8}O_{4}.

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.

So, M

**Step 2: **Change
the grams to moles for salicylic acid

So, 100 g = 100 ÷ 138 mole = 0.725 moles

**Step 3: **Work
out the calculated mass of the 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.

So, the percent yield = 121.2 ÷ 130.5 × 100% = 92.9%

Example:

Consider a 3.52-g sample of CaCO

This chemistry tutorial cover the difference between actual, theoretical and percent yields and include examples of how to calculate theoretical and percent yields.

The

The

Example:

If 16.0 g of CaCO

Step 1: Always check to make sure the equation is balanced.

Step 2: Calculate the theoretical yield of CaO.

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 O

a) What is the limiting reagent?

b) What is the theoretical yield of H

c) Experiment gives 10g H

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:

*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

CaCO_{3} (*s*) + 2HCl (*aq*) → CaCl_{2}
(*aq*) + H_{2}O (*l*) + CO_{2} (*g*)

*Solution: *

**Step 1:**
Calculate the M_{r} of calcium carbonate

M

**Step 2: **Calculate
the grams from the volume

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

Percent purity = 9.5 ÷ 10 × 100% = 95%

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?

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.

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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