Conquer the AP Chemistry Free Response Section

The free response section on the AP Chemistry exam can seem incredibly daunting. Especially because the 95-minute, six-multi-part free response accounts for half of your score! (The other half will come from a 90-minute, 75-question multiple choice section.)

The Bad News: With short answer and essay, you can’t hide your lack of studying behind smart elimination skills or playing the probability odds like you can with the multiple choice questions.

The Good News: Partial Credit is a beautiful thing!  Free response questions allow you the opportunity to still get quite a few points even if you do not get the right final answer. And, there are some good test-taking strategies to maximize the number of points you can obtain on the free response section.

The Breakdown:

The AP chemistry free response section is broken down into two parts. Part A has 3 multi-part quantitative questions that you will need to answer in fifty-five minutes. This is the only part of the AP Chemistry exam that allows you to use a calculator. You can count on one of the three questions to be an equilibrium problem, which means, if you think Le Chatelier is a brand of gourmet chocolates, you might want to brush up on your equilibrium skills.

Part B has 3 multi-part qualitative questions that must be completed in forty minutes. You are not allowed to use a calculator here, but that is not a problem because most of these questions involve translating chemistry stuff into English. One of the questions will always involve writing and balancing three chemical reactions.

Helpful Tips:

The free response section is scored by several graders who are looking for certain things in every problem. Here are some things to keep in mind to maximize your score:

1) Show all of your work. You can receive partial credit, so write something, anything, down. Write down any formulas from the formula sheet that you know you will need to use. Write down the reactants to the chemical reaction, even if you have no idea what the products will be. Show your calculations, even if you punched them in the calculator. The graders need to see inside your head, so show them what you are doing!

2) Attempt every part of the problem. Even if you did not get the answer to part “b” of a multi-part problem, still go on to parts “c” and “d.” You will receive some credit for good reasoning, even if your numbers are off.

3) Write neatly. The graders can’t give you credit for something they can’t read.

4) Pay attention to units and significant figures. For full credit on a problem, be sure to use the correct units and the correct significant figures.

Let’s take a look at a typical quantitative problem.


If the Ka of acetic acid is 1.8 x 10-5 at a certain temperature, what is the pH of 0.01M acetic acid?

The pH of an acid is based on the concentration of the H+ ion in solution, but acetic acid is weak acid so it does not completely dissociate in solution. That means we need to use the equilibrium constant, Ka, and some R-I-C-E (Reaction, Initial Conditions, Change, Equilibrium) to figure out how much H+ we have.

Use R-I-C-E to solve equilibrium problems:

Write the chemical REACTION:

HC2H3O2 + H2O à C2H3O2 + H3O+

Write the INITIAL CONDITIONS of each species in the reaction:

HC2H3O2 + H2O à C2H3O2 + H3O+

0.01M         n/a         0            0

Write the CHANGE that must occur for this reaction to go to equilibrium:

HC2H3O2 + H2O à C2H3O2 + H3O+

0.01M         n/a         0            0

0.01 – x                   +x              +x

Write the EQUILIBRIUM equation:

Ka = [C2H3O2] [H3O+] / [HC2H3O2]

We can substitute what we know into the equilibrium equation and solve for x.

1.8 x 10-5 = (x) (x) / (0.01-x)

For a weak acid: 0.01 – x ≈ 0.01

x  = [H3O+] = 4.2 x 10-4

We know that pH = -log [H3O+], so –log [4.2 x 10-4] is 3.4.

Maybe you forgot that pH is the negative log of H­3O+ or you accidentally made a math error when solving for x. That’s okay, you still would have received some credit for this problem, and in the end, those points add up!

If you need support in your AP Chemistry class check out AP Companion Courses from Revolution Prep:

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