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3.1 EQUATIONS AND MASS RELATIONSHIPS

A balanced chemical equation such as

4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g) (3.1)

not only tells how many molecules of each kind are involved in a reaction, it also indicates the amount of each substance that is involved. Equation (3.1) says that 4NH₃ molecules can react with 5O₂ molecules to give 4 NO molecules and 6H₂O molecules. It also says that 4 mol NH₃ would react with 5 mol O₂ yielding 4 mol NO and 6 mol H₂O.

The balanced equation does more than this, though. It also tells us that 2 × 4 = 8 mol NH₃ will react with 2 × 5 = 10 mol O₂, and that ½ × 4 = 2 mol NH₃ requires only ½ × 5 = 2.5 mol O₂. In other words, the equation indicates that exactly 5 mol O₂ must react for every 4 mol NH₃ consumed. For the purpose of calculating how much O₂ is required to react with a certain amount of NH₃ therefore, the significant information contained in Eq. (3.1) is the ratio

$\frac{\text{5 mol O}_{\text{2}}}{\text{4 mol NH}_{\text{3}}}$

We shall call such a ratio derived from a balanced chemical equation a stoichiometric ratio and give it the symbol S. Thus, for Eq. (3.1),

$\text{S}\left( \frac{\text{O}_{\text{2}}}{\text{NH}_{\text{3}}} \right)=\frac{\text{5 mol O}_{\text{2}}}{\text{4 mol NH}_{\text{3}}}\text{ (3}\text{.2)}$

The word stoichiometric comes from the Greek words stoicheion, “element,“ and metron, “measure.“ Hence the stoichiometric ratio measures one element (or compound) against another.

EXAMPLE 3.1 Derive all possible stoichiometric ratios from Eq. (3.1)

Solution Any ratio of amounts of substance given by coefficients in the equation may be used:

$\text{S}\left( \frac{\text{NH}_{3}}{\text{O}_{\text{2}}} \right)=\frac{\text{4 mol NH}_{3}}{\text{5 mol O}_{\text{2}}}$ $\text{S}\left( \frac{\text{O}_{\text{2}}}{\text{NH}} \right)=\frac{\text{5 mol O}_{\text{2}}}{\text{4 mol NH}}$

$\text{S}\left( \frac{\text{NH}_{3}}{\text{NO}} \right)=\frac{\text{4 mol NH}_{3}}{\text{4 mol NO}}$ $\text{S}\left( \frac{\text{O}_{\text{2}}}{\text{H}_{\text{2}}\text{O}} \right)=\frac{\text{5 mol O}_{\text{2}}}{\text{6 mol H}_{\text{2}}\text{O}}$

$\text{S}\left( \frac{\text{NH}_{3}}{\text{H}_{\text{2}}\text{O}} \right)=\frac{\text{4 mol NH}_{3}}{\text{6 mol H}_{\text{2}}\text{O}}$ $\text{S}\left( \frac{\text{NO}}{\text{H}_{\text{2}}\text{O}} \right)=\frac{\text{4 mol NO}}{\text{6 mol H}_{\text{2}}\text{O}}$

There are six more stoichiometric ratios, each of which is the reciprocal of one of these. [Eq. (3.2) gives one of them.]

When any chemical reaction occurs, the amounts of substances consumed or produced are related by the appropriate stoichiometric ratios. Using Eq. (3.1) as an example, this means that the ratio of the amount of O₂ consumed to the amount of NH₃ consumed must be the stoichiometric ratio S(O₂/NH₃):

$\frac{n_{\text{O}_{\text{2}}\text{ consumed}}}{n_{\text{NH}_{\text{3}}\text{ consumed}}}=\text{S}\left( \frac{\text{NH}_{3}}{\text{O}_{\text{2}}} \right)=\frac{\text{4 mol NH}_{3}}{\text{5 mol O}_{\text{2}}}$

Similarly, the ratio of the amount of[[Image:]]produced to the amount of[[Image:]]consumed must be

S(H₂O/NH₃):

$\frac{n_{\text{H}_{\text{2}}\text{O produced}}}{n_{\text{NH}_{\text{3}}\text{ consumed}}}=\text{S}\left( \frac{\text{H}_{\text{2}}\text{O}}{\text{NH}_{3}} \right)=\frac{\text{6 mol H}_{\text{2}}\text{O}}{\text{4 mol NH}_{3}}$

In general we can say that

$\text{Stoichiometric ratio }\left( \frac{\text{X}}{\text{Y}} \right)=\frac{\text{amount of X consumed or produced}}{\text{amount of Y consumed or produced}}\text{ (3}\text{.3a)}$

or, in symbols,

$\text{S}\left( \frac{\text{X}}{\text{Y}} \right)=\frac{n_{\text{X consumed or produced}}}{n_{\text{Y consumed or produced}}}\text{ (3}\text{.3}\text{.b)}$

Note that in the word Eq. (3.3a) and the symbolic Eq. (3.3b), X and Y may represent any reactant or any product in the balanced chemical equation from which the stoichiometric ratio was derived. No matter how much of each reactant we have, the amounts of reactants consumed and the amounts of products produced will be in appropriate stoichiometric ratios.

EXAMPLE 3.2 Find the amount of water produced when 3.68 mol NH₃ is consumed according to Eq. (3.1).

Solution The amount of water produced must be in the stoichiometric ratio S(H₂O/NH₃) to the amount of ammonia consumed:

$\text{S}\left( \frac{\text{H}_{\text{2}}\text{O}}{\text{NH}_{\text{3}}} \right)=\frac{n_{\text{H}_{\text{2}}\text{O produced}}}{n_{\text{NH}_{\text{3}}\text{ consumed}}}$

Multiplying both sides n_{NH3 consumed}, by we have

$n_{\text{H}_{\text{2}}\text{O produced}}=n_{\text{NH}_{\text{3}}\text{ consumed}}\times \text{S}\left( \frac{\text{H}_{\text{2}}\text{O}}{\text{NH}_{\text{3}}} \right)=\text{3}\text{.68 mol NH}_{\text{3}}\times \frac{\text{6 mol H}_{\text{2}}\text{O}}{\text{4 mol NH}_{\text{3}}}=\text{5}\text{.52 mol H}_{\text{2}}\text{O}$

This is a typical illustration of the use of a stoichiometric ratio as a conversion factor. Example 3.2 is analogous to Examples 1.10 and 1.11, where density was employed as a conversion factor between mass and volume. Example 3.2 is also analogous to Examples 2.4 and 2.6, in which the Avogadro constant and molar mass were used as conversion factors. As in these previous cases, there is no need to memorize or do algebraic manipulations with Eq. (3.3) when using the stoichiometric ratio. Simply remember that the coefficients in a balanced chemical equation give stoichiometric ratios, and that the proper choice results in cancellation of units. In road-map form

$\text{amount of X consumed or produced}\overset{\begin{smallmatrix} \text{stoichiometric} \\ \text{ ratio X/Y} \end{smallmatrix}}{\longleftrightarrow}\text{amount of Y consumed or produced}$

or symbolically.

$n_{\text{X consumed or produced}}\text{ }\overset{S\text{(X/Y)}}{\longleftrightarrow}\text{ }n_{\text{Y consumed or produced}}$

When using stoichiometric ratios, be sure you always indicate moles of what. You can only cancel moles of the same substance. In other words, 1 mol NH₃ cancels 1 mol NH₂ but does not cancel 1 mol H₂O.

The next example shows that stoichiometric ratios are also useful in problems involving the mass of a reactant or product.

EXAMPLE 3.3 Calculate the mass of sulfur dioxide (SO₂) produced when 3.84 mol O₂ is reacted with FeS₂ according to the equation

4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂

Solution The problem asks that we calculate the mass of SO₂ produced. As we learned in Sec. 2.8, Example 2.6, the molar mass can be used to convert from the amount of SO₂ to the mass of SO₂. Therefore this problem in effect is asking that we calculate the amount of SO₂ produced from the amount of O₂ consumed. This is the same problem as in Example 3.2. It requires the stoichiometric ratio

$\text{S}\left( \frac{\text{SO}_{\text{2}}}{\text{O}_{\text{2}}} \right)=\frac{\text{8 mol SO}_{\text{2}}}{\text{11 mol O}_{\text{2}}}$

The amount of SO₂ produced is then

$n_{\text{SO}_{\text{2}}}=n_{\text{SO}_{\text{2}}\text{ consumed}}\text{ }\!\!\times\!\!\text{ conversion factor}=\text{3}\text{.84 mol O}_{\text{2}}\times \frac{\text{8 mol SO}_{\text{2}}}{\text{11 mol O}_{\text{2}}}=\text{2}\text{.79 mol SO}_{\text{2}}$

The mass of SO₂ is

$\text{m}_{\text{SO}_{\text{2}}}=\text{2}\text{.79 mol SO}_{\text{2}}\times \frac{\text{64}\text{.06 g SO}_{\text{2}}}{\text{1 mol SO}_{\text{2}}}=\text{179 g SO}_{\text{2}}$

With practice this kind of problem can be solved in one step by concentrating on the units. The appropriate stoichiometric ratio will convert moles of O₂ to moles of SO₂ and the molar mass will convert moles of SO₂ to grams of SO₂. A schematic road map for the one-step calculation can be written as

$n_{\text{O}_{\text{2}}}\text{ }\xrightarrow{S\text{(SO}_{\text{2}}\text{/O}_{\text{2}}\text{)}}\text{ }n_{\text{SO}_{\text{2}}}\text{ }\xrightarrow{M_{\text{SO}_{\text{2}}}}\text{ }m_{\text{SO}_{\text{2}}}$

Thus

$\text{m}_{\text{SO}_{\text{2}}}=\text{3}\text{.84 mol O}_{\text{2}}\times \text{ }\frac{\text{8 mol SO}_{\text{2}}}{\text{11 mol O}_{\text{2}}}\text{ }\times \text{ }\frac{\text{64}\text{.06 g}}{\text{1 mol SO}_{\text{2}}}=\text{179 g}$

The chemical reaction in this example is of environmental interest. Iron pyrite (FeS₂) is often an impurity in coal, and so burning this fuel in a power plant produces sulfur dioxide (SO₂), a major air pollutant.

Our next example also involves burning a fuel and its effect on the atmosphere.

EXAMPLE 3.4 What mass of oxygen would be consumed when 3.3 × 10¹⁵ g, 3.3 Pg (petagrams), of octane (C₈H₁₈) is burned to produce CO₂ and H₂O?

Solution First, write a balanced equation

2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O

The problem gives the mass of C₈H₁₈ burned and asks for the mass of O₂ required to combine with it. Thinking the problem through before trying to solve it, we realize that the molar mass of octane could be used to calculate the amount of octane consumed. Then we need a stoichiometric ratio to get the amount of O₂ consumed. Finally, the molar mass of O₂ permits calculation of the mass of O₂. Symbolically

$m_{\text{C}_{\text{8}}\text{H}_{\text{18}}}\text{ }\xrightarrow{M_{\text{C}_{\text{8}}\text{H}_{\text{18}}}}\text{ }n_{\text{C}_{\text{8}}\text{H}_{\text{18}}}\text{ }\xrightarrow{S\text{(SO}_{\text{2}}\text{/C}_{\text{8}}\text{H}_{\text{18}}\text{)}}\text{ }n_{\text{O}_{\text{2}}}\xrightarrow{M_{\text{O}_{\text{2}}}}\text{ }m_{\text{O}_{\text{2}}}$

$m_{\text{O}_{\text{2}}}=\text{3}\text{.3 }\times \text{ 10}^{\text{15}}\text{ g }\times \text{ }\frac{\text{1 mol C}_{\text{8}}\text{H}_{\text{18}}}{\text{114 g}}\text{ }\times \text{ }\frac{\text{25 mol O}_{\text{2}}}{\text{2 mol C}_{\text{8}}\text{H}_{\text{18}}}\text{ }\times \text{ }\frac{\text{32}\text{.00 g}}{\text{1 mol O}_{\text{2}}}=\text{1}\text{.2 }\times \text{ 10}^{\text{16}}\text{ g }$

Thus 12 Pg (petagrams) of O₂ would be needed.

The large mass of oxygen obtained in this example is an estimate of how much O₂ is removed from the earth’s atmosphere each year by human activities. Octane, a component of gasoline, was chosen to represent coal, gas, and other fossil fuels. Fortunately, the total mass of oxygen in the air (1.2 × 10²¹ g) is much larger than the yearly consumption. If we were to go on burning fuel at the present rate, it would take about 100 000 years to use up all the O₂. Actually we will consume the fossil fuels long before that! One of the least of our environmental worries is running out of atmospheric oxygen.

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