Chapter 9

Percent Yield

Theoretical Yield vs. Actual Yield• Theoretical yield is the amount of product we should produce from

the complete conversion of a given amount of reactant to product – must be calculated

• Actual yield is the amount of product that is actually formed during a reaction. – it is always less than theoretical yield because of incomplete reactions,

impure reactants, side reactions, etc– must be given within the problem or measured during an experiment.

• Theoretical yield is calculated from the amount of limiting reactant.

Making Popcorn

• If we started with 80 popcorn kernels and found only 72 of them popped.

• What is the percent yield of popcorn?

Percent Yield

actual yield% yield = x 100%

theoretical yield

A reaction was supposed to make was 2.50 grams of Zn. You actually made 2.32 grams. What is the percent yield?

2Al + 3CuCl2 → 3Cu + 2AlCl3• A chemistry student is trying to prepare copper metal

by the reaction of aluminum with copper (II) chloride.

→

2Al + 3CuCl2 → 3Cu + 2AlCl3• If the student is able to prepare 3.67g of copper metal

by the reaction of 1.27g of aluminum with excess copper (II) chloride. What is the percent yield?

6. Chromium (III) hydroxide will dissolve in concentrated sodium hydroxide according to the following equation:

NaOH + Cr(OH)3 NaCr(OH)4

If you begin with 66.0g of Cr(OH)3 and obtain 38.4g of NaCr(OH)4, what is the percentage yield?

Two other types of percent yield problems

• In both these problems you will be given the percent yield.

1. Given the percent yield determine the amount of a product you can produce.

– You make less.

2. Given the percent yield determine the amount of a reactant you need.

– You use more.

You make less• A bakery produces

cookies at a percent yield of 92%. The bakers add enough ingredients to make 1525 cookies. How many cookies do they produce?

1525 (0.92) = 1403 cookies

Multiply by the percent yield. “You make Less”

You use more.

• The same bakery (92% yield) has an order placed for 2150 cookies. The bakers will need to use enough ingredients for how many cookies.

2150 0.92

= 2337 cookies

Divide by percent yield. “You use more”

3. Ammonia is synthesized from hydrogen and nitrogen according to the following equation:

N2 + 3H2 2NH3

If an excess of nitrogen is reacted with 3.41g of hydrogen, how many grams of ammonia can be produced assuming the reaction has a percentage yield of 48.8%?

9. A chemical engineer needs to produce 212 g of zinc oxide by the process below:

2ZnS + 3O2 2ZnO + 2SO2

By doing trials runs he expects the process to run at a 78% yield. How much zinc sulfide should he use to prepare the 212 g of zinc oxide?

Homework

• Percentage Yield Worksheet