Percent Yield and Percent Purity

Now that we've covered the Excess and Limiting Reactants, we can move onto calculating other things, such as Percent Yield.

As we learned in the Excess blog, sometimes not all reactants are used or not all of the product is recovered.

That is why we're introducing this new topic: % yield.

Percent yield is a ratio of the product obtained to the product from calculation, expressed as a percentage.

The formula (not really a formula, but the way of calculating it) is:

g from actual experiment        X 100%      
g from calculations (expected)



But here's a helpful note: your answer should never be higher than 100%.

Now, let's try an example.

Say we have the reaction:

CH4 + O2 ---> CO2 + H2O

If 10.2 g of CH4 reacted with sufficient O2, and the actual yield of CO2 was 23.0 g, what is the percent yield of this reaction?

First, we balance the equation.

CH4 + 2O2 ---> CO2 + 2H2O

Now, we use stoichiometry calculations to get the theoretical yield (how much CO2 we should be getting)

10.2 g CH4 X 1 mole CH4 X 1 mole CO2 X 44.0 g CO2  = 28.0 g CO2
                             16.0 g         1 mole CH4      1 mole CO2

So, that means, theoretically, this reaction should produce 28.0 g of CO2. However, our actual yield was only 23.0 g.

So now, we divide.    23.0 g CO2 X 100% = 82.0%
                                   28.0 g CO2
That is, for every 100.0 g of product predicted, only 82.0g is actually formed.

Next, let's talk about Percent Purity.

Sometimes, the reactants we use are not pure.
So Percent purity is the ratio of the mass of the pure substance to the mass of the impure sample, expressed as a percentage.

This is very similar to the percent yield, and to calculate percent purity, we use g pure    X 100%
                                                                                                                            g impure

Let's try an example.

1.00g Fe ore contains 0.6g Fe metal. What is the percent purity?



0.6/1.0 X 100% = 60 %

If you still have some troubles, watch this video and do some problems!
http://www.uen.org/utahlink/tours/tourElement.cgi?element_id=42288&tour_id=17891&category_id=33176
http://misterguch.brinkster.net/PRA022.pdf