Law of Multiple Proportions: Videos & Practice Problems

The Law of Multiple Proportions, formulated by John Dalton in 1804, is a fundamental concept in chemistry that describes how elements combine in simple whole number ratios to form different compounds. When examining compounds like nitrogen monoxide (NO) and nitrogen dioxide (NO₂), one can apply this law by calculating the mass ratios of the elements involved. By determining the atomic mass of nitrogen and oxygen from the periodic table, multiplying by the number of atoms in each compound, and comparing the mass ratios, it's possible to see that NO and NO₂ obey the law. For instance, NO has a mass ratio of 1.14 (16 g of oxygen to 14.1 g of nitrogen), while NO₂ has a mass ratio of 2.28 (32 g of oxygen to 14.1 g of nitrogen). Dividing the larger mass ratio by the smaller one yields a whole number, confirming that these compounds adhere to the Law of Multiple Proportions. This whole number ratio is indicative of the law's validity and reflects the predictable ways in which elements combine to form compounds.

Law of Multiple Proportions

The law of multiple proportions, formulated by the English chemist John Dalton in 1804, is a fundamental principle in chemistry that states when two elements, A and B, combine to form different compounds, the masses of element B that combine with a fixed mass of element A can be expressed as a ratio of whole numbers. This concept is best understood through practical examples.

Consider the compounds nitrogen monoxide (NO) and nitrogen dioxide (NO₂). To illustrate how these compounds adhere to the law of multiple proportions, we can follow a systematic approach:

First, we need to determine the atomic masses of nitrogen and oxygen from the periodic table. Nitrogen has an atomic mass of approximately 14.01 grams per mole, while oxygen has an atomic mass of about 16 grams per mole.

Next, we calculate the total mass of each compound. For nitrogen monoxide (NO), which consists of one nitrogen atom and one oxygen atom, the total mass is:

Mass of NO = 14.01 g/mol (N) + 16.00 g/mol (O) = 30.01 g/mol

For nitrogen dioxide (NO₂), which contains one nitrogen atom and two oxygen atoms, the total mass is:

Mass of NO₂ = 14.01 g/mol (N) + 2 × 16.00 g/mol (O) = 46.01 g/mol

Now, we can determine the mass ratios of oxygen to nitrogen for each compound. For NO, the mass ratio is:

Mass ratio (NO) = 16 g (O) / 14.01 g (N) ≈ 1.142

For NO₂, the mass ratio is:

Mass ratio (NO₂) = 32 g (O) / 14.01 g (N) ≈ 2.284

To verify adherence to the law of multiple proportions, we take the ratio of the two mass ratios. We place the larger mass ratio on top and the smaller on the bottom:

Ratio = 2.284 / 1.142 ≈ 2

Since this result is a whole number, it confirms that nitrogen monoxide and nitrogen dioxide follow the law of multiple proportions. This illustrates that the same elements can combine in different ways to form distinct compounds, with NO representing a 1:1 ratio of nitrogen to oxygen, while NO₂ represents a 1:2 ratio.

Here’s what students ask on this topic:

What is the law of multiple proportions?

The law of multiple proportions is a fundamental principle in chemistry, which states that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers. This law was first articulated by John Dalton in the early 19th century, and it is a key piece of evidence for the atomic theory of matter.

For example, consider carbon and oxygen. They can combine to form carbon monoxide (CO) and carbon dioxide (CO₂). If you take 12 grams of carbon to react with oxygen to form carbon monoxide, you'll need 16 grams of oxygen. For carbon dioxide, those same 12 grams of carbon would combine with 32 grams of oxygen. The ratio of the mass of oxygen in carbon dioxide to the mass of oxygen in carbon monoxide is 32:16, which simplifies to 2:1, a small whole number ratio. This consistent ratio reflects the different proportions in which elements can combine to form distinct compounds.

What does the law of multiple proportions say?

The law of multiple proportions is a fundamental principle in chemistry, which states that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in the ratio of small whole numbers. For example, consider carbon and oxygen forming carbon monoxide (CO) and carbon dioxide (CO₂). If you have 12 grams of carbon in both compounds, in carbon monoxide, it might combine with 16 grams of oxygen, while in carbon dioxide, it would combine with 32 grams of oxygen. The ratio of the mass of oxygen in carbon dioxide to the mass of oxygen in carbon monoxide is 32:16, which simplifies to a small whole number ratio of 2:1. This law was put forth by John Dalton in the early 19th century and is a crucial concept for understanding the stoichiometric relationships in chemical reactions.

Which of the following is an example of the law of multiple proportions?

The law of multiple proportions states that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other are in the ratio of small whole numbers.

An example of this law is the formation of carbon oxides. Carbon can react with oxygen to form two different compounds: carbon monoxide (C1O1) and carbon dioxide (C1O2). In carbon monoxide, for every 12 grams of carbon, there are 16 grams of oxygen. In carbon dioxide, for the same 12 grams of carbon, there are 32 grams of oxygen.

If you compare the amount of oxygen in both compounds, the ratio is 16:32 or simplified to 1:2. This demonstrates that for a fixed amount of carbon, the mass of oxygen combines in a ratio of small whole numbers (1:2 in this case), which is consistent with the law of multiple proportions.