Percent Yield and Percent Purity:

Hello, everyone!!!!

Today we study percentage yield and percentage purity. I wish you have a good journey here.

Part 1

What is percent yield?

The yield is the amount of product you obtain from a reaction.

Example:

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(The substances are products from the reactiom, so it is the percentage yield.)


The formula of percent yield is:

*2chem55.gif

Percent yield calculations have three kinds:

To calculate the percent yield, use the reactant mass and product mass fill in the formula of percent yield.

To calculate the mass of product, use the mass of reactant and the percent yield fill in the formula.

To calculate mass of reactant, use the mass of product and the percent yield fill in the formula.



*Example:


The medical drug aspirin is made from salicylic acid. 1 mole of salicylic acid gives 1 mole of aspirin. Given that the chemical formular 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 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

138g of salicylic acid = 1 mole

So, 100g = 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 × 180g = 130.5g

So, the calculated mass of the reaction is 130.5g

Step 4: Calculate the percent yield.

The actual mass obtained is 121.2g

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

Part 2

What is Percent Purity?

The percent of a specified compound or element in an impure substances.

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(these two products are the same, but they have different percent purity, so their color look different)

The formula of percent purity is:

*4chem56.gif

Percent purity calculations also have three kinds:

To calculate the percent purity, use the reactant mass and product mass fill in the formula of percentage yield.

To calculate the mass of product formed, use the mass of reactant and the percent yield fill in the formula.

To calculate the mass of reactant used, use the mass of product and the percent yield fill in the formula.

*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 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 = 9.5g

Step 3: Calculate the percent purity

There is 9.5g of calcium carbonate in the 10g of chalk.

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

LINK:

*2,4 http://www.onlinemathlearning.com/percent-yield.html