1.1 What is Chemistry?

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

Mole Ratios: Equating Changes in Amount

We previously saw that mass is conserved for any chemical reaction. Atoms present during the beginning of a
chemical reaction must be present at the end as well, even though they may be arranged in different ways. Consider
the reaction between iron and oxygen to produce iron(III) oxide:

4Fe(s) + 3O 2 (g)→2Fe 2 O 3 (s)

According to this equation 4 moles of iron will react with 3 moles of oxygen gas (O 2 ) to produce 2 moles of iron(III)
oxide, Fe 2 O 3 , as a product. Of course, we do not need exactly 4 moles of iron or 3 moles of oxygen for this reaction
to occur. Rather, this equation tells us the ratio in which these reactants combine to make a particular product.

When we express the relative amounts of two reaction components as a ratio, we refer to this as amole ratioor a
stoichiometric ratio. Mole ratios can be made between two reactants, two products, or one of each. For example, the
following mole ratios can be obtained by looking at the balanced equation shown above:

4 mol Fe

3 mol O 2 or

3 mol O 2

4 mol Fe

4 mol Fe

2 mol Fe 2 O 3 or

2 mol Fe 2 O 3

4 mol Fe

3 mol O 2

2 mol Fe 2 O 3 or

3 mol O 2
2 mol Fe 2 O 3
Stoichiometryrefers to the calculations involving mole ratios to determine the relative amounts of reactants needed
to produce a given amount of product. Consider the reaction of sodium chloride with silver nitrate:

AgNO 3 (aq)+ NaCl(aq)→AgCl(s)+ NaNO 3 (aq)

Silver chloride is an important compound that is commonly used in the production of photographic film. It also has
many other uses, such as an antidote for mercury poisoning, a component of pottery glazes, and a reference standard
for electrochemistry setups. It can be produced according to the reaction shown above. Now we will practice use of
mole ratios and stoichiometry to determine the amounts of products and reactants necessary in our reaction.

Example 12.1

How many moles of each reactant are needed to produce 0.5 mol of silver chloride?

Answer:

For this problem, we need to relate moles of each reactant to moles of the product silver chloride. The mole ratio of
silver nitrate to silver chloride is constructed as follows:

As shown above, we would need 0.5 mol of silver nitrate to produce 0.5 mol of silver chloride.

We could also express the moles of silver chloride in terms of sodium chloride, our other reactant. Here is how we
would do this: