Chemical Calculations Lesson # 3 – Percent Composition

The Law of Definite Proportions The Law states that the composition of a specific compound is constant. This means, for example, that a water molecule will always contain two hydrogen atoms and one oxygen atom. This is true anywhere in the universe. If it had three hydrogen atoms instead, it would not be water, but another substance all together. This law is usually expressed in ratios, for example, there will always be 2 hydrogen for every 1 oxygen, or in mass ratios, stating there will always be 8 g of oxygen compared to 1 g of hydrogen. This leads us to the discussion of percent composition, which is used instead of stating the actual mass of each element in a compound – it uses percentage values to represent the composition of the compound by mass. To do this we compare the mass of each type of element in a sample to the total mass of a sample. This works, because regardless of the actual amount used in the lab, the percentages will always be the same – they should always add up to 100%.

Example 1 Calculate the percent composition by mass of carbon monoxide.

Example 2 Calculate the percent composition by mass of nitric acid.

Example 3 Calculate the percentage composition of sucrose (C11H22O11).

Mass Spectrometer A mass spectrometer is used to measure the molar mass of a compound. A small sample of the compound is vaporized (turned into gas) and bombarded by a beam of electrons. The electrons cause the molecules to become electrically charged (ionized) and possibly broken up into a number of smaller fragments. The molar mass of the original molecule can be determined from the molar mass of the largest fragment.

Combustion Analyzer A combustion analyzer is used to determine the percentages of carbon, hydrogen, oxygen, and possibly nitrogen in a compound. An accurately known mass of a pure compound is burned in a stream of pure oxygen gas at a temperature of approximately 980°C. Oxygen is used because it is required for the combustion of carbon-based compounds. All the carbon in the sample is converted into carbon dioxide gas. All the hydrogen in the same is converted into water vapour (steam). All the metals in the sample are converted into their oxides (e.g. iron would change to iron oxide).