1.1 What is Chemistry?

http://www.ck12.org Chapter 12. Stoichiometry

Note: The change in the number of moles for each reactant and product must be consistent with the mole ratios in the
balanced chemical equation. This is designated by the factor of "x" in the Change in Moles row of the reaction table.
Also recall that as a chemical reaction takes place, the reactants are being used up (indicating a negative change in
moles) and the products are being created (indicating a positive change in moles).

Steps to Solving Chemical Reaction Problems

1. Write the balanced reaction.


2. Draw a reaction table.


3. Fill in the known values.


4. Calculate the missing values.

This process is easiest to explain in the form of an example problem.

Example 12.7

Magnesium metal is heated in the presence of oxygen gas to produce magnesium oxide.

2Mg(s)+ O 2 (g)→2MgO(s)

If 1.00 grams of Mg react completely with excess oxygen, how many grams of magnesium oxide will be produced?
How many grams of oxygen will be used?

Answer:

First, write the balanced reaction, and then draw a blank reaction table. The changes in moles of each component
can be written in terms of a variable x, which we will solve for. The relative number of moles added to or subtracted
from each amount is based on the coefficients from the balanced equation.

2Mg(s) + O 2 (g)→2MgO(s)

TABLE12.2:Magnesium Table Set Up

Mg O 2 MgO

Molar Mass

Initial Mass

Initial Moles

Change in Moles -2x -x +2x

Final Moles

Final Mass

Now, fill in the known values. We are told that the initial mass of magnesium is 1.00 grams. We are also told that
our magnesium sample reacts completely, so the final mass of magnesium (and the final moles) will be 0. We do not
know exactly how much oxygen gas is present, but it is anexcess reactant(sometimes called an excess reagent),
which means that there is more than enough oxygen to react with the other available reactants. We can simply write
"excess" in both the initial and final masses/moles for this reactant. Unless told otherwise, assume that there is no
initial product (the initial mass and moles of MgO would be 0). Additionally, we can calculate the molar masses for
each reactant and product by looking at the periodic table:

Mg = 24.31 g/mol

O 2 = 32.00 g/mol

MgO = 40.31 g/mol