AP Chemistry : Calculations

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

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Example Question #31 : Laboratory Techniques And Analysis

Calculate and give the answer with the correct number of significant figures.

Possible Answers:

Correct answer:

Explanation:

For multistep calculations, make sure to keep track of the number of significant figures at the end of each step so we know how many significant figures to round to at the end of the entire calculation. To maintain accuracy, do not round intermediate steps.

First, do the two multiplication problems found within the parentheses.

Recall that for multiplication, the result needs to have the same number of significant figures as the factor with the least number of significant figures. Our answer from the addition should then only have 3 significant figures since that is the number of significant figures 3.14 has. To remember that the answer should only have 3 significant figures, the last significant digit will be highlighted: 

Recall that for multiplication, the result needs to have the same number of significant figures as the factor with the least number of significant figures. Our answer from the addition should then only have 1 significant figure since that is the number of significant figures 10 has. To remember that the answer should only have 1 significant figure, the last significant digit will be highlighted: 

Now, multiply:

Since the factor with the least number of significant figures only has 1 significant figure, the answer must also be rounded to 1 significant figure. 5570.13393 is rounded to 6000.

Example Question #31 : Calculations

Calculate and give the answer to the correct number of significant figures.

Possible Answers:

Correct answer:

Explanation:

For multistep calculations, make sure to keep track of the number of significant figures at the end of each step so we know how many significant figures to round to at the end of the entire calculation. To maintain accuracy, do not round intermediate steps.

First, do the two multiplication problems found within the parentheses.

Recall that for multiplication, the result needs to have the same number of significant figures as the factor with the least number of significant figures. Our answer from the addition should then only have 1 significant figure since that is the number of significant figures 0.02 has. To remember that the answer should only have 3 significant figures, the last significant digit will be highlighted: 

Next, do the addition.

Because this is addition, the result of the calculation needs to have the same number of decimal places as the number with the fewest decimal places. We need to only keep 1 decimal place because of . The last significant digit from this step is highlighted: 

Next, do the second addition.

Because this is addition, the result of the calculation needs to have the same number of decimal places as the number with the fewest decimal places. Since 10 does not have any decimal places, our final answer also cannot have any decimal places. 33.356 is rounded down to 33.

Example Question #33 : Laboratory Techniques And Analysis

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 #1 : 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 #1 : 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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