Page 154 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 10] STOICHIOMETRY 143

but we used the one above because it is the one that changes moles of oxygen to moles of aluminum oxide.
2 mol Al 2 O 3 2 mol Al 2 O 3 3 mol O 2 3 mol O 2 4 mol Al 4 mol Al
3 mol O 2 4 mol Al 4 mol Al 2 mol Al 2 O 3 3 mol O 2 2 mol Al 2 O 3
EXAMPLE 10.2. How many moles of oxygen does it take to react completely with 1.48 mol of aluminum?
Ans. According to the equation in Example 10.1, it takes

3 mol O 2
1.48 mol Al = 1.11 mol O 2
4 mol Al
EXAMPLE 10.3. How many moles of NO 2 are produced by the reaction at high temperature of 1.50 mol of O 2 with
sufficient N 2 ?
Ans. The balanced equation is

N 2 + 2O 2 −→ 2NO 2

2 mol NO 2
1.50 mol O 2 = 1.50 mol NO 2
2 mol O 2
A simple figure linking the quantities, with the factor label as a bridge, is shown in Fig. 10-1.

Balanced chemical
Moles A Moles B
equation
Fig. 10-1. The conversion of moles of one reagent to moles of another, using a ratio of the coefficients of the balanced
chemical equation as a factor label

In all the problems given above, a sufficient or excess quantity of a second reactant was stated in the problem.
If nothing is stated about the quantity of a second (or third, etc.) reactant, it must be assumed to be present in
sufficient quantity to allow the reaction to take place. Otherwise, no calculation can be done.

EXAMPLE 10.4. How many moles of NaCl are produced by the reaction of 0.750 mol Cl 2 (with Na)?
Ans. We must assume that there is enough sodium present. As long as we have enough sodium, we can base the calculation
on the quantity of chlorine stated.

2Na + Cl 2 −→ 2 NaCl

2 mol NaCl
0.750 mol Cl 2 = 1.50 mol NaCl
1 mol Cl 2

10.2. CALCULATIONS INVOLVING OTHER QUANTITIES
The balanced equation expresses quantities in moles, but it is seldom possible to measure out quantities in
moles directly. If the quantities given or required are expressed in other units, it is necessary to convert them to
moles before using the factors of the balanced chemical equation. Conversion of mass to moles and vice versa
was considered in Sec. 7.4. First we will use that knowledge to calculate the number of moles of reactant or
product and use that value to calculate the numbers of moles of other reactants or products.

EXAMPLE 10.5. How many moles of Na 2 SO 4 are produced by reaction of 124 g of NaOH with sufficient H 2 SO 4 ?
Ans. Again, the first step is to write the balanced chemical equation:
2 NaOH + H 2 SO 4 −→ Na 2 SO 4 + 2H 2 O