We know that Antione Lavoisier investigated combustion reactions which led to the Law of Mass Conservation. Joseph Proust (1754 – 1826), a French chemist, was able to formulate another fundamental law called the Law of Definite Proportions (also referred to as the Law of Definite Composition). According to the law, a chemical compound always contains its component elements in a fixed ratio, by mass, regardless of its source or preparation. For example, water (H₂O), regardless of its source, always contains 2 parts hydrogen to 1 part oxygen, by mass. Hydrogen peroxide (H₂O₂) always contains 1 part hydrogen to 1 part oxygen by mass. Ammonia (NH₃) will always contain 1 part nitrogen to 3 parts hydrogen, by mass.

To summarize, the component elements in a compound combine in specific proportions. Carbon monoxide (CO) is composed of 1 part carbon to 1 part oxygen while carbon dioxide (CO₂) is always 2 parts oxygen to 1 part carbon by mass.

The Law of Multiple Proportions was proposed by John Dalton (1766 – 1844). It was observed that some elements such as carbon and oxygen can form more than one compound, CO and CO₂. We see the ratio of oxygen to carbon is different for both compounds. The Law of Multiple Proportions states that if two elements combine to form more than one compound, the mass of one element that combines with a fixed mass of another element is always in ratios of small whole numbers.

We can perform an experiment where we combine 40.0 g of carbon with 30.0 g of oxygen to produce CO. Under different experimental conditions, 40.0 g of carbon is reacted with 60.0 g of oxygen, and CO₂ is produced. The ratio of the two masses of oxygen is 30:60 or 1:2, a ratio of small whole numbers. Another example is the formation of two compounds of sulfur and oxygen, SO₂ and SO₃. In one experiment, 10.0 g of sulfur will react with 10.0 of oxygen and SO₂ is produced. Again, if experimental conditions are changed, 10.0 g of sulfur reacts with 15.0 g of oxygen to form SO₃. Here we see a 10:15 (2:3), ratio of the two oxygen masses. Again, this illustrates the Law of Multiple Proportions.

Worksheet: Laws of Mass Conservation and Multiple Proportions

Exercises

Exercise 1. Carbon suboxide is a compound made up of carbon and oxygen. There is also carbon monoxide, CO, and carbon dioxide, CO₂. If a 5.00 g sample of carbon suboxide contains 2.64 g of C and 2.36 g of O, show the law of multiple proportions is adhered to. What is a possible formula for carbon suboxide?

Check Solution/Answer to Exercise 1

Exercise 2. Two compounds that contain nitrogen and oxygen have the following percent composition by mass.

Compound 1: 30.4% N and 69.6% O
Compound 2: 22.6% N and 77.4% O

Show the law of multiple proportions is followed. What is the formula of the second compound if the formula of the first compound is NO₂?

Check Solution/Answer to Exercise 2

Exercise 3. A sample of 22.65 g of silicon tetrachloride, SiCl₄, reacts with 10.25 g of water, H₂O. The products, silicon dioxide and HCl, have a total mass of 27.45 g. How much water is left unreacted?

Check Solution/Answer to Exercise 3

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Study Guide List for General Chemistry 1