5 Steps to a 5 AP Chemistry

Experimental

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

cal equation. Typical experiments involving these concepts are 1, 2, 6, 7, 8, 9, 16, and 17

in the Experimental chapter.

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 soluteper 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

Answer the following questions. You have 25 minutes, and you may not use a calculator.

You may use the periodic table in the back of the book. For each question, circle the letter

of your choice.

Stoichiometry  95

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

(E) 60.0 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

(E) 0.5424 M

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