AP Chemistry : Yield and Error

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

Example Question #1 : Yield And Error

3 mols of  are placed in a reactor and react to form  as in the above reaction. At equilibrium, there are 1.75 mols of .

What is the percent conversion of ?

Possible Answers:

Correct answer:

Explanation:

To find the percent conversion, use the following equation:

 

Initially there are 3 mols of . We know that every 1 mol of  that is consumed in the reaction yields 1 mol of . Therefore, the final amount of  in mols is

Plugging this into the above equation gives a conversion rate of 58%.

Example Question #2 : Yield And Error

You run an experiment in order to empirically find the gas constant, .  You find 

You know from the literature that

What is the percent error in your experimental value of ?

Possible Answers:

Correct answer:

Explanation:

To find the percent error in your  value, you can use the following equation:

Doing this, we find the percent error to be 4.6%.

Example Question #3 : Yield And Error

Suppose that a chemist working in a lab is trying to synthesize caffeine. In doing so, he predicts that he can produce  of caffeine. After the chemist finishes synthesizing the caffeine, he weighs it only to find out that  are produced. What is the percent yield of caffeine in this synthesis process?

Possible Answers:

Correct answer:

Explanation:

When chemists synthesize compounds in the lab, a theoretical yield can be calculated by using knowledge of the full, balanced chemical equation, as well as the starting amounts of all reagents including the limiting reagent. However, due to a variety of factors, the actual yield obtained at the end of the experiment is nearly always going to be lower than the theoretical yield. Some of the things that can cause this are experimental error and side reactions, among others. Thus, chemists usually take the ratio of actual yield to theoretical yield to give the percent yield.

This means that the overall process was  efficient.

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