5 Steps to a 5 AP Chemistry 2019

Stoichiometry ‹ 95

Experiments

Stoichiometry experiments must involve moles. They nearly always use a balanced chemi-

cal equation. Measurements include initial and final masses, and initial and final volumes.

Calculations may include the difference between the initial and final values. Using the for-

mula mass and the mass in grams, moles may be calculated. Moles may also be calculated

from the volume of a solution and its molarity.

Once the moles have been calculated (they are never measured), the experiment will be

based on further calculations using these moles.

Common Mistakes to Avoid

1. Avogadro’s number is 6.022 × 1023 (not 10-^23 ).


2. Be sure to know the difference between molecules and moles.


3. In empirical formula problems, be sure to get the lowest ratio of whole numbers.


4. In stoichiometry problems, be sure to use the balanced chemical equation.


5. The stoichiometric ratio comes from the balanced chemical equation.


6. When in doubt, convert to moles.


7. In limiting-reactant problems, don’t consider just the number of grams or even moles
   to determine the limiting reactant—use the mol/coefficient ratio.


8. The limiting reactant is a reactant, a chemical species to the left of the reactant arrow.


9. Use the balanced chemical equation.


10. Percent yield is actual yield of a substance divided by the theoretical yield of the same
    substance multiplied by 100%.


11. Molarity is moles of solute per liter of solution, not solvent.


12. Be careful when using Avogadro’s number—use it when you need or have the number of
    atoms, ions, or molecules.

❯ Review Questions

Use these questions to review the content of this chapter and practice for the AP Chemistry

exam. First are 18 multiple-choice questions similar to what you will encounter in Section I

of the AP Chemistry exam. Following those is a long free-response question like the ones

in Section II of the exam. To make these questions an even more authentic practice for the

actual exam, time yourself following the instructions provided.

Multiple-Choice Questions

Answer the following questions in 25 minutes. You may not use a calculator. You may use

the periodic table and the equation sheet at the back of this book.

1. How many milliliters of 0.100 M H 2 SO 4 are
   required to neutralize 50.0 mL of 0.200 M
   KOH?
   (A) 25.0 mL
   (B) 30.0 mL
   (C) 20.0 mL
   (D) 50.0 mL
   2. A sample of oxalic acid, H 2 C 2 O 4 , is titrated with
   standard sodium hydroxide, NaOH, solution.
   A total of 45.20 mL of 0.1200 M NaOH is
   required to completely neutralize 20.00 mL of
   the acid. What is the concentration of the acid?
   (A) 0.2712 M
   (B) 0.1200 M
   (C) 0.1356 M
   (D) 0.2400 M

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