Chemical Calculation Problems for the AP Test

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  • 0:06 What to Expect
  • 1:56 Tips for Performing…
  • 4:09 Practice Multiple Choice
  • 5:19 Practice Free Response…
  • 9:18 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

Learn the types of calculations you are likely to perform on the AP Chemistry test. Then look at some tips and tricks for enhancing your performance. Learn how to approach sample questions from former AP tests, apply tips and tricks you've just learned!

What to Expect

What words come to mind when you think of chemical calculations?

Maybe equations? Units? Dimensional analysis? Stoichiometry?

Equations, calculator, decimal, scientific notation, units, formulas, dimensional analysis, exponents, significant figures, stoichiometry...

All of these terms - and many more - are important when performing chemical calculations.

If you were thinking of negative words or ideas, stop that! You can totally do chemical calculations. You are an AP student with all the tools and knowledge to figure out what a problem is asking, how you can logically solve it and show your work.

You will encounter chemical calculations on both the multiple choice section and free response section of the AP Chemistry test. You cannot use a calculator for the multiple choice section. You can use a calculator for the free response section. There are seven multi-step free response questions. Calculations will occur as part of multi-step problems within the free response section.

Prior to heading into the test you should be able to:

  • Determine percent composition or mass percent
  • Determine empirical or molecular formula from data
  • Determine molar mass from gas density, freezing point or boiling point data
  • Perform calculations with gas laws, including Dalton's law and Graham's Law
  • Perform stoichiometric calculations involving mole ratios
  • Find concentrations of unknown solutions from titrations
  • Determine mole fractions
  • Perform calculations with molar solutions
  • Use Faraday's law of electrolysis
  • Perform thermodynamics and thermochemical calculations
  • Any calculation related to kinetics
  • Perform calculations related to equilibrium constants (especially Ka, Kb and Ksp)
  • Perform data analysis calculations, like percent error and percent yield

Tips for Performing Chemical Calculations

Your strategy for performing chemical calculations will vary from the multiple choice section to the free response section. There are a few study habits that are useful in preparing for either section.

Study the equations and constants chart

In fact, print one off now! You can go to the AP Central Chemistry Exam website and access the exact same equations and constants chart that you'll be given on the exam. Go through all the equations and constants and become familiar with what they are, what they can be used for and the appropriate units to use. Keep this as your loyal study companion. There are some important formulas that are not on this sheet. Make sure you note them as you study.

Pay attention to units

Not only are units necessary for answers, they can give you hints as to what equation (or equations) you may need to use.

Pay attention to significant figures

Learn what numbers are not helpful and what numbers are crucial to include in your answer. This can save time and decrease frustration.

One of the most important things to do is understand what you are calculating and why. Having an understanding of what is going on will help you choose the right formula and give you a sense if your answer is right.

On the multiple choice section, you will want to pace yourself. Give yourself about 40 seconds per question. If you can't solve the question in that time, move on. Return to the question if you have left over time.

Also on the multiple choice section, you will have to estimate, approximate, and round because you will not have a calculator. Tune up your algebra skills in preparation for this. Practice operations involving decimals, exponents, and logarithms. Learn how to quickly convert between common units like: mL --> L, g --> Kg.

You can scribble and do work all over the multiple choice questions, but you will only be graded on the answers you put on the answer sheet.

For the free response section, you will also want to pace yourself. Spend about 16 minutes on each long question and 10 minutes on the short questions.

It is imperative on this section that you show your work. You get partial credit for work that you do. Sometimes you get points for work that wasn't asked for on the prompt. Show you work, label your intermediates with correct units, present your work in an organized fashion.

Practice Multiple Choice

Here's an example multiple choice question from the 2012 course description: Note that this example provides five answer options. The new test format offers only four.

A solution prepared by mixing 10 mL of 1 M HCl and 10 mL of 1.2 M NaOH has a pH of:

(a) 0 (b) 1 (c) 7 (d) 13 (e) 14

Because I don't have a calculator, it's in my best interest to approximate, estimate and eliminate wrong answers as early on as possible. I do have access to a equations and constants chart which I can use if I get stuck or need some inspiration.

I know based on information from the prompt that I will have excess NaOH, which will make my solution more basic. Since my solution is more basic, then I can eliminate answers (a), (b), and (c).

The next step is to do a little math and determine if my answer is (d) or (e). If I needed help remembering how to determine pH based on OH- concentration, I'd look at my equations and constants chart.

Kw = [H+][OH-] = 1.0 x 10^-14, pH = -log [H+], pOH = log [OH-], 14 = pH + pOH

Using some of these equations and my sharp math skills, I estimate my pH to be about 13. That means my answer is (d).

Practice Free Response Calculations

The following question came from the 2013 AP test:

Answer the following questions involving the stoichiometry and thermodynamics of reactions containing aluminum species.

2 Al2O3(l) + 3 C(s) --> 4 Al(l) + 3 CO2(g)

An electrolytic cell produces 235 g of Al(l) according to the equation above.

(a) Calculate the number of moles of electrons that must be transferred in the cell to produce the 235 g of Al(l).

(b) A steady current of 152 amps was used during the process. Determine the amount of time, in seconds, that was needed to produce the Al(l).

(c) Calculate the volume of CO2(g), measured at 301 K and 0.952 atm, that is produced in the process.

I will do part (a) and part (b) as examples. If you want to test yourself, do part (c) by yourself. I will give the answer at the end of this segment.

To answer questions like these, I always start by listing my given information and my desired information.

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