Lesson 4.08

1. Lesson 4.08 Percent Yield

2. Introduction Percentages are often used to show productivity or success. • What are some real-life examples that involve percentages? • What do they have in common? • What are they being used to represent?

3. Objectives After completing this lesson, you will be able to: Calculate the percent yield in a reaction

4. Real-Life Examples Have you ever gotten a numerical grade on a test? How do you determine the letter grade that you’ve earned? Imagine that you had a grade of 63 out of 75. Divide the score that you earned by the maximum possible points that you could have earned. Then multiply that decimal value by 100% to make it a percentage. 63/75 × 100% = 84% (Congrats you got a B!)

5. Real-Life Examples In sports, percentages are often used as a way to communicate how well an athlete is performing. A quarterback’s completion percentage in American football is the number of completed passes divided by the total number of passes attempted. # completed/# attempted x 100= completion percentage The higher his percentage, the more successful a quarterback has been at throwing passes that were completed, or caught, by his teammates.

6. A Common Theme As you can see, percentages can be used to report progress, productivity, or success in a variety of areas, including grades and sports.

7. What do you already know? • The theoretical yield is the maximum amount of the product able to be produced based on your given amounts of reactants. • The limiting reactant is the reactant or “ingredient” that limits how much of the product you can make. (it runs out first) • Use stoichiometry to determine the theoretical yield.

8. Actual Yield The measured amount of product actually obtained from a reaction is the actual yield. The actual yield (amount of product made) is usually less than the theoretical yield (amount that could have been made)

9. Why? Some possible reasons why the actual yield is lower than the theoretical yield: • Conditions do not allow the reaction to run to completion • Purification results in loss of some of the product

10. Efficiency Chemists are often concerned with the efficiency of a reaction, or how the actual yield relates to the theoretical, or maximum, yield. The percent yield of a reaction is the ratio of the actual yield to the theoretical yield, multiplied by 100 to make it a percentage. Percent yield = Actual x 100 Theoretical

11. Example Imagine that you’re baking chocolate chip cookies and you have enough dough made to bake five dozen cookies. Your best friend calls while you have a cookie sheet in the oven, and one dozen cookies end up burning because you were distracted. If you throw those burned cookies aside, or feed them to your little brother, you only have four dozen cookies that turned out What is your percent yield, or the percentage of the cookies that were baked successfully?

12. Answer You could have made a maximum of five dozen cookies (theoretical yield), but only were able to successfully produce four dozen cookies (actual yield). 4 dozen cookies (actual yield) 80% = X 100 5 dozen cookies (theoretical yield)

13. Steps for Calculating Percent Yield • Write the balanced equation for the reaction. • Identify all important information provided. • Solve for the theoretical yield using a stoichiometry calculation. • Divide the actual yield (the measured amount of product produced) by the theoretical yield (the calculated amount of product from the stoichiometry calculation) and multiply by 100 to get the percent yield

14. Example 1 9.80 grams of magnesium metal reacts with excess hydrochloric acid, and 29.8 grams of magnesium chloride are produced by the reaction. What is the percent yield of this reaction? Mg + HCl → MgCl2 + H2 NOTE: “excess” means you have more than you need

15. Solving Example 1~ Step 1 • Write the balanced equation for the reaction. Mg + 2 HCl → MgCl2 + H2

16. Solving Example 1 ~Step 2 Identify all important information provided. 9.80 grams of magnesium metal reacts with excess hydrochloric acid, and 29.8 grams of magnesium chloride are produced by the reaction. • 9.80 grams of magnesium metal is the given amount of one reactant, and the other reactant (HCl) is in excess. This means that it is not a limiting reactant problem, you will only need to do one stoichiometry calculation to determine theoretical yield. • 29.8 grams of magnesium chloride is the actual yield that was collected and measured in the lab.

17. Solving Example 1 ~Step 3 Solve for the theoretical yield using stoichiometry Limiting Reactant Mole/Mole Ratio Molar Mass

18. Solving Example 1 ~Step 4 Calculate the Percent Yield

19. Example 2 ~ Your Turn! When 70.0 grams of MnO2 reacted with 128.0 grams of HCl, the reaction resulted in a 62.7 percent yield of chlorine gas. What is the actual yield of chlorine gas in grams? MnO2 + HCI → MnCl2 + H2O + Cl2

20. Steps to Solve Example 2 • What is the balanced equation? • What is the important information? • What is the theoretical yield? (two stoichiometry problems) • What is the actual yield? (rearrange the equation)

21. Check your Answers • Balanced equation MnO2 + 4 HCI → MnCl2 + 2 H2O + Cl2 • What is the important information? 70.0 g MnO2 and 128.0 g HCl; percent yield of Cl2 is 62.7 percent • What is the theoretical yield? 57.2 g Cl2 • What is the actual yield? 35.9 g Cl2

22. How did you do? Solving for theoretical yield

23. Solving for actual yield

24. Review

25. Congratulations! You should now be able to: Determine the percent yield of a reaction and use the percent yield to solve for the actual yield For more practice problems, please visit the lesson in the course