AP Chemistry · Exam guide
AP Chemistry Lab and Data Questions Explained: Graphs, Beer's Law and Sources of Error
Long free-response questions often describe a laboratory procedure and ask you to use the data, explain the method, or say how a mistake would change the result. The same few ideas appear again and again. The data below are made up for practice.
A worked calibration line (Beer's law)
A student measures the absorbance of four solutions of a colored compound at known concentrations in a 1.0 cm cell.
| Concentration (mM) | Absorbance |
|---|---|
| 0.10 | 0.12 |
| 0.20 | 0.24 |
| 0.30 | 0.36 |
| 0.40 | 0.48 |
- Beer's law says absorbance is proportional to concentration, so a graph of absorbance against concentration is a straight line through the origin.
- The slope is 0.12 / 0.10 = 1.2 per mM, which equals the molar absorptivity times the path length.
- An unknown solution has an absorbance of 0.30. Its concentration is 0.30 / 1.2 = 0.25 mM.
Notice that the unknown is within the range of the standards. A concentration read from beyond the line is an extrapolation and is less reliable.
Explaining the effect of an error
In a titration of an acid with a base, a student passes the endpoint, so too much base is added. How does this error affect the calculated concentration of the acid?
It does not say in which direction or why.
It follows the error through the calculation to a direction.
What lab questions usually ask
| Request | How to answer |
|---|---|
| Identify a measurement or piece of equipment | Name it and what it measures, such as a buret for the volume of base delivered. |
| Describe a procedure step | Say what is done and why it is needed, such as rinsing a buret with the solution it will hold. |
| Interpret a graph | State what the slope or intercept represents, with units. |
| Explain an error | Trace it through the calculation and give a direction: too high or too low. |
Common mistakes
- Saying that an error makes the result "inaccurate" without a direction.
- Blaming "human error". The question wants a specific cause.
- Forgetting units on the slope or the final answer.
- Drawing a best-fit line that connects the points instead of passing through the middle of them.
- Using a calibration line outside the range of the standards.
Frequently asked questions
What is Beer's law?
It says the absorbance of a solution is proportional to its concentration, as long as the path length stays the same.
Why do labs use a calibration line?
It lets you convert a measurement you can make, such as absorbance, into a quantity you cannot measure directly, such as concentration.
How do I say how an error changes the result?
Follow it through the calculation. Ask which measured quantity became too large or too small, and what that does to the answer.
Do I need to know specific lab equipment?
Know what the common items measure: a buret, a pipet, a balance, a thermometer, a spectrophotometer.
Where can I find the official scoring rules?
Exam formats change a little from year to year, so check the current course and exam description on the College Board site before test day.
Keep practicing
Every lesson in the AP Chemistry course ends with practice items and an Exam focus box, and every unit has a ten-question practice set with worked answers. These free lessons also build the skills the free-response section tests:
See the AP Chemistry course Free lessons and lesson list
Other free-response guides:
- AP Chemistry FRQ Guide: Long and Short Questions, Units and Particle Explanations
- AP Chemistry Equilibrium Question Explained: ICE Tables, Q Versus K and Le Chatelier
- AP Chemistry Acid-Base and Buffer Question Explained: pH, Ka and the Henderson-Hasselbalch Equation
- AP Chemistry Particle-Level Explanations: How to Explain Trends, Boiling Points and Bonding
AP is a registered trademark of the College Board, which is not affiliated with and does not endorse this site. This guide is our own explanation of the question types and scoring. For the official scoring guidelines, see the College Board.