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We (the authors) have been grading the free-response part of the AP Chemistry exam for
quite a while. Between the two of us, we have nearly 20 years of grading experience—that’s
more than 100,000 exams! Over the years, we have seen quite a number of careless mistakes
made by students. These mistakes resulted from not being careful rather than not being pre-
pared for the exam. Here are some practical tips to avoid the most common careless errors.
• Don’t forget to state the units of measurement. Many students would have gotten
more credit if they had shown the units, both in the calculations and in the final answer.
The units help you stay on the right track and help the grader determine if (or where)
you went wrong.
• Use the formula given. If the exam gives you a chemical formula, don’t use a different
formula in your answer. In general, do not alter anything given to you on the exam. For
example, we have seen Ba(NO 3 ) 2 become Ba(NO 2 ) 2.
• Be careful with the math. We have seen many errors involving the simplest math such
as 12 mL + 3 mL = 0.042 L (rather than 0.015 L).
• Don’t confuse molarity and moles. The units M and [ ] are identical (molarity) and
are completely different from moles.
• Show your work for conversions. For example, if you are changing grams to moles and
make a simple mistake, showing your work (labeled) may get you partial credit.
• Don’t argue with the test. This is an argument you cannot win. For example, if the question
asks for calculations, you are unlikely to get full credit without any calculations even if you
have the right answer. It won’t help to write that you feel the calculations are unnecessary.
• Be careful in applying gas laws. Gas laws can be very useful. However, they should
never be used when there is not a gas in the problem. Having a volume included in the
question information doesn’t necessarily mean you are dealing with a gas.
• Be careful making comparisons. We have seen many students incorrectly say that 10–8
is smaller than 10–12 and actually write 10–8 < 10–12. We have even seen students write
the relationship correctly (10–8 > 10–12) but still state that 10–8 is smaller.
• Be careful using 22.4 L/mol. You will probably not need to use this on the exam. But
if you do want to use this value, you must have a gas and this gas must be at 0°C (273 K) and
1 atm (STP). If you forget the values for STP, they can be found on the exam. We have
seen quite a few students incorrectly use this value at 298 K.
• There are no trick questions on the exam. If you think you have found a trick ques-
tion, you need to reevaluate your thinking and reread the question.
• Don’t confuse solutions and precipitates in solution. They are different phases and
are not interchangeable. The color of one is not necessarily the color of the other.
• Be careful describing reactions. If the problem gives you, for example, a sodium nitrate
solution, part of your answer describing a reaction cannot be “the sodium nitrate dis-
solves.” You already have a solution, so the process of dissolving happened before you
got to the problem. Furthermore, dissolving should not be treated as a reaction.
• Be careful using positive and negative charges. In the following equation, each reac-
tant and product is wrong: NH 4 + NO 3 → NH 4 +NO 3 – , and will not substitute for theAVOIDING “STUPID” MISTAKES ON
THE FREE-RESPONSE SECTION
22-Moore_APP_p371-412.indd 393 31/05/18 1:59 pm