Percent Yield and Purity

First, let's look at Percent Yield.

Sometimes not all the product is recovered or all the reactants are not used up, so we calculate the Percent Yield. The basic formula is: amount of product obtained / amount of product expected X 100 to get percent.
Basically, when dividing the two numbers, if you get something over 1, something, somewhere, has gone terribly wrong...

Now let's use this info in an example:
15.0 g of CH4 reacts with excess Cl2 according to the following equation:
CH4 + Cl2 = CH3Cl + HCl
a total of 29.7 grams of CH3Cl is produced. What is the percentage yield?

First, we have to find the mass of CH3Cl that is expected:
15.0 g CH4 X 1 mol CH3Cl / 16.0g CH4 X 1 mol CH3Cl / 1 mol CH4 X 50.5 g CH3Cl / 1 mol CH3Cl = 47.34 g

Now let's figure out the percentage yield:
29.7 g / 47.34 g X 100 = 62.7 %
*Remember not to round until the end and use sig. figs.*

Simple enough, right? Let's do a backwards one to see if you really get it:
What mass of CuO is required to make 10.0 g of Cu according to the reaction:
2NH3 + 3CuO = N2 + 3Cu + 3H2O
if the reaction has a 58.0% yield?

We have to find the mass of CuO:
10.0 g Cu X 1 mol / 63.5 g Cu X 3 mol CuO / 3 mol Cu X 79.5 g CuO / 1 mol CuO = 12.52 g

Now, we have to divide this amount by 0.580 to get the LARGER number that will allow for the LOSS that will occur during the reaction:
12.52 g / 0.580 = 21.6 g


One more example:
What mass of K2CO3 is produced when 1.50 g of KO2 is reacted with an excess of CO2 according to the reaction:
4KO2 + 2CO2 = 2K2CO3 + 3O2
if the reaction has a 76.0% yield?

Calculate the mass of K2CO3:
1.50 g KO2 X 1 mol KO2 / 71.1 g KO2 X 2 mol K2CO3 / 4 mol KO2 X 138.2 g K2CO3 / 1 mol K2CO3 = 1.458 g

Now multiply that by the percent yield:
1.458 g X 0.760 = 1.11 g


Phew! Any more questions? Watch this video:

If you have this down, the next part will be easy.

What's more pure than a sparkly diamond?

Percentage Purity Time!

Only the pure part of a substance will actually react so sometimes we need to calculate the percentage purity of a substance.
The formula is basically the same as percentage yield: mass of pure reactant / mass of impure reactant X 100

Example:
If 100.0 g of FeO produces 12.9 g of pure Fe according to the reaction:
2FeO + 2C + O2 = 2Fe + 2CO2
what is the percentage purity of FeO used?

Same as before, find the mass of FeOl:
12.9 g Fe X 1 mol Fe / 55.8 g Fe X 2 mol FeO / 2 mol Fe X 71.8 g FeO / 1 mol FeO = 16.6 g

Calculate the percentage purity:
16.6 g / 100.0 g X 100 = 16.6 %


As you can see, everything is basically the same as percentage yield. Got it? Good. Have fun!