Chemistry: Stoichiometry

The atomic ratios in each compound are also the relative number of atomic weight units of its elements. The first example is nitrous oxide (N₂O), as shown in Table 1 .

TABLE 1

Nitrous Oxide

*Chemical Element*	*Atomic Ratio*	*Atomic Relative*	*Relative Weight*	*Weight Percent*
Nitrogen	2	14.01	28.02	63.65
Oxygen	1	16.00	16.00	36.35

The relative weights were obtained by multiplying the atomic ratios and atomic weights. You can see that a sample of N₂O weighing 44.02 grams contains 28.02 g of nitrogen and 16.00 g of oxygen. The weight percent of each element is calculated from its relative weight divided by the sum of the relative weights. Chemical compounds with integral atomic ratios, like nitrous oxide, are described as stoichiometric compounds, and they permit many simple calculations.

The common oxide of aluminum provides a second example, but this time, begin with the weight percent and deduce the atomic ratio. Careful laboratory analysis of aluminum oxide determines it to be approximately 53% aluminum and 47% oxygen by weight, as shown in the second column in Table 2 .

TABLE 2

Aluminium Oxide

*Chemical Element*	*Atomic Ratio*	*Atomic Relative*	*Relative Weight*	*Weight Percent*
Aluminium	2	26.98	1.9161	52.92
Oxygen	3	16.00	2.943	47.08

The relative weights are obtained by dividing the weight percentages by the atomic weights from the periodic table.

So, the ratio of oxygen to aluminum in relative atomic weights is

If the aluminum oxide is a stoichiometric compound with whole-number ratios of the constituent elements, the preceding quotient must be translated into a ratio of integers:

This ratio of integers implies that the formula for aluminum oxide is A1₂O₃. In simple compounds, the metallic elements are written before the nonmetals.

Most chemical compounds are stoichiometric, and you should be able to utilize the atomic weights and perform the following calculations:

Weight percents from the chemical formula
Chemical formula from the weight percents

These calculations are so basic to the field that you should go back and carefully review the two examples in this section: the calculation of weight percents in N₂O and the inference of the formula for aluminum oxide. Then you can practice stoichiometric calculations on the following pair of problems.

Problem 1: Determine the weight percentages of the three elements in ammonium chloride, which has the formula NH₄C1.
Problem 2: Infer the simplest chemical formula for potassium copper fluoride, which has this analysis by weight: