CK-12-Chemistry Intermediate

12.2. Stoichiometric Calculations http://www.ck12.org

Step 2: Solve.

4 .60 mol SO 2 ×

3 mol O 2

2 mol SO 2

×

32 .00 g O 2

1 mol O 2

=221 g O 2

Step 3: Think about your result.

According to the mole ratio, 6.90 mol O 2 is produced, which has a mass of 221 g. The answer has three significant
figures because the given value has three significant figures.

Practice Problem

2. Copper metal reacts with sulfur to form copper(I) sulfide. What mass of copper(I) sulfide is produced by the
   reaction of 0.528 mol Cu with excess sulfur?

Mass to Mass Problems

Mass-mass calculations are the most practical of all mass-based stoichiometry problems. Moles cannot be measured
directly, but masses can be easily measured in the lab for most substances. This type of problem is three steps and is
a combination of the two previous types.

mass ofgiven→moles ofgiven→moles ofunknown→mass ofunknown

The mass of the given substance is converted into moles by using its molar mass. Then, the moles of the given
substance are converted into moles of the unknown by using the mole ratio from the balanced chemical equation.
Finally, the moles of the unknown are converted back to a mass by using its molar mass.

Sample Problem 12.5: Mass-Mass Stoichiometry

Ammonium nitrate decomposes to dinitrogen monoxide and water according to the following equation.

NH 4 NO 3 (s)→N 2 O(g) + 2H 2 O(l)

In a certain experiment, 45.7 g of ammonium nitrate is decomposed. Find the mass of each of the products formed.

Step 1: List the known quantities and plan the problem.

Known

• given: 45.7 g NH 4 NO 3


• 1 mol NH 4 NO 3 = 1 mol N 2 O = 2 mol H 2 O (mole ratios)


• molar mass of NH 4 NO 3 = 80.06 g/mol


• molar mass of N 2 O = 44.02 g/mol


• molar mass of H 2 O = 18.02 g/mol

Unknown

• mass N 2 O =? g


• mass H 2 O =? g

Perform two separate three-step mass-mass calculations, as shown below.