Chapter 5: Unit 9. Percent Yield

Percent Yield

It is often important to calculate percent yield in a chemical reaction to estimate the efficiency of a chemical reaction. Percent yield is defined as

Percent yield= (actual/theoretical)*100%. The actual yield is the amount of product in a chemical reaction, determined by weighing a product on a balance. Theoretical yield is a quantity calculated from a balanced chemical equation, using mole ratios and molar masses. The theoretical yield is the maximum amount of product formed in a chemical reaction from the amount of reactants used.

When we perform stoichiometric calculations to determine the amount of product produced – that is a theoretical yield. But when we actually perform the experiment in a lab setting, we usually find we do not get as much product, we usually get a smaller actual yield due to human error or other experimental error.

To calculate the percent yield, the actual yield and theoretical yield are needed. You prepared cookie dough to make 5 dozen cookies. The phone rings and you answer. While talking, a sheet of 12 cookies burn and you have to throw them out. The rest of the cookies are okay. What is the percent yield of edible cookies?

Theoretical yield 60 cookies possible

Actual yield 48 cookies to eat

Percent yield 48 cookies x 100 = 80% yield

Examples:

1. the reaction of carbon and oxygen produces carbon monoxide. 2C(g) + O 2 (g) …..> 2CO(g) What is the percent yield if 40.0 g CO are actually produced when 30.0 g O 2 are used? (Need to find the theoretical 1 st
   1) 25.0% 2) 75.0% 3) 76.2%

Ans: theoretical yield of CO 30.0 g O 2 x 1 mol O 2 x  28.01 g CO   = 52.5 g CO (theoretical)
2 mol CO

percent yield 40.0 g CO (actual) x 100 = 76.2 % yield 52.5 g CO (theoretical)

2. When N ₂ and 5.00 g H ₂ are mixed, the reaction produces 16.0 g NH 3. What is the percent yield for the reaction? N ₂ (g) + 3H ₂ (g) → 2NH ₃ (g)
   1) 31.3 % 2) 56.5 % 3) 80.0 %

N ₂ (g) + 3H ₂ (g) →2NH ₃ (g)

5.00 g H 2 x 1 mol H₂          x 2 mol NH ₃   x 17.03 g NH 3= 28.2 g NH ₃ (theoretical)
2.106 g H₂        3 mols H₂

Percent yield = 16.0 g NH₃ x 100 = 56.7 % 28.2 g NH ₃ 

The following activity has been taken from AACT

Cookie Stoichiometry

Stoichiometry is a process in which the equation for a chemical reaction is used like a recipe.  We will do mass to mass problems, limiting reactant problems, and percent yield problems just like we did with this cookie recipe.  It will be a piece of cake!!

Original Nestle Tollhouse Cookie Recipe Ingredients:

2 1/4 cups all-purpose flour

1 teaspoon baking soda

1 teaspoon salt

1 cup (2 sticks) butter or margarine, softened

3/4 cup granulated sugar

3/4 cup packed brown sugar

1 teaspoon vanilla extract

2 large eggs

2 cups (12 oz. pkg.) NESTLÉ TOLL HOUSE Semi-Sweet Chocolate Morsels

PREHEAT oven to 375° F.  COMBINE flour, baking soda and salt in small bowl. Beat butter, granulated sugar, brown sugar and vanilla extract in large mixer bowl until creamy. Add eggs one at a time, beating well after each addition. Gradually beat in flour mixture. Stir in morsels and nuts. Drop by rounded tablespoon onto ungreased baking sheets. BAKE for 9 to 11 minutes or until golden brown. Cool on baking sheets for 2 minutes; remove to wire racks to cool completely.

11 minutes, 375° Makes 60 cookies.

Use the above cookie recipe to answer the following questions.  Be sure to show your work and cross out units.

1. Write 3 ratios that can be used from this recipe.

2. If you make 45 cookies, how many batches is this?

3. How many cups of brown sugar are needed to completely “react” with 8 eggs?

4. How many batches of cookies can be made from 1 cup of morsels?

5. If you start out with 4 eggs and 3 cups of flour, which one will run out first? This is called the “limiting reactant”.

6. Using the information from #5, how many batches can you make if you have plenty of the other ingredients?

7. Which ingredient did you use to get the answer to #6, the eggs or the flour?

8. How many cookies is this?

9. If you had enough ingredients to make a batch of cookies and you made 53 cookies (you probably ate the rest of the dough!), what is your percent yield?

Questions:

1. In a particular reaction, the theoretical yield is 37.4 g, If the actual yield is 31.2g. what is the percent yield?

2. 2C₂H₅OH(l) → (C₂H₅)₂O(l) + H₂O(l)

In the above reaction, 46.0 g of C₂H₅OH produces 35.9 g diethyl ether. What is the percent yield?

Ans: 1. 83.4% 2. 96.9%