Percent Yield and Stoichiometry

Stoichiometry

• In the reaction between 5.5g magnesium and an excess amount of silver nitrate how many grams of silver should I produce?

• When I actually performed the experiment 45.3g of silver was isolated.

• What percent of the amount I should have had did I actually end up with?

Why didn’t I recover 100% of the product?

• Reactions do not always work perfectly. In any given experiment there will be sources of experimental error. Because of this, the amount of product made will not usually be the same as the calculated amount.

Definitions

• Theoretical Yield: The maximum amount of product that can be formed in a reaction – determined using stoichiometry. – 48.8 g of Ag in previous example.

• Actual Yield: The amount of product that is actually formed/isolated from a chemical reaction – this is a measured value.– 45.3 g of Ag in previous example.

Percent Yield

• Percent Yield: The amount of product that was recovered relative to the theoretical (calculated) amount.

Percent Yield = Actual Yield x 100

Theoretical Yield

Example

• When I was in second year at university, we had a lab where we were supposed to isolate caffeine from tea leaves. Most of my caffeine was accidentally washed down the drain. Although I should have had 5.0 g of caffeine, I only ended up with 0.040 g of caffeine and a bad grade. What was my percent yield?

Example

• Johnny was sloppy and rushing so he did not wait for his PbI2 to dry before measuring its mass. As a result, his product weighed 2.25g when it should have been 1.75g. What was his percent yield?

Example

• What is the theoretical yield if 5.50g of hydrogen react with nitrogen to form ammonia?

• If 51.6214g of ammonia is produced, what is my percent yield?

A Big Example

• 5.3 mol of calcium and 3.8 mol of aluminum chloride are added to the reaction flask. a. Are these quantities in stoichiometric amounts?

If not determine the limiting reagent.b. What is the theoretical yield of aluminum in

grams?c. 90.521g of aluminum was isolated. What is the

percent yield?

Homework

Read: Section 7.5

Questions: p.338 # 1-2

p. 399 # 6, 7, 8, 13.