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

In this video we'll take a look at the law of multiple proportions. So here in 1804 the English chemist John Dalton originated the Law of multiple proportions based on his atomic theory. Now it states that when two elements A&B form different compounds, the mass of element B that combined with 1g of a are a ratio of whole numbers. Now, a definition like this isn't exactly clear, so in order to better understand the law of multiple proportions, it's best to jump right into an example.

So let's take a look here at this example question. Here it says illustrate how nitrogen monoxide, which is NO, and nitrogen dioxide, which is NO₂, obey the law of multiple proportions. All right, so when we're given a question dealing with the law of multiple proportions, we're going to follow these steps. So step one, we're going to find the atomic masses of each element from the periodic table. Nitrogen monoxide and nitrogen dioxide both contains nitrogen and oxygen. Nitrogen has an atomic mass of 14.01g per mole. Oxygen has roughly atomic mass of 16g per mole.

What we do next is we multiply together the number of each element with their atomic masses from the periodic table. Nitrogen monoxide is NO, which is just one nitrogen and one oxygen, so that comes out to 14.01g per mole for nitrogen and 16.0g per mole for oxygen. NO₂ has in it 1 nitrogen and two oxygens, so that would come out to 14.01g per mole of nitrogen and 32g per mole of oxygen. Step three, we have to determine the mass ratios of each compound. So remember when it comes to mass ratio you should place the element with the larger atomic mass on top. So for NO that would be the 16 grams of oxygen for the every 14.01 grams of nitrogen. That comes out to 1.142 as their mass ratio. And then for NO₂, that would be 32 grams oxygen over 14.01g nitrogen. So that will come out to 2.284.

All right. So we know at this point we've figured out the mass ratios of both. Step 4 is what shows us that they follow the law of multiple proportions. Now Step 4, if the law of multiple proportions is being followed, then the ratio of both mass ratios should give a whole number. So we're going to place the larger mass ratio on top divided by the smaller mass ratio on the bottom. When we do that, we get a value of two because we've gotten a whole number. That means that NO and NO₂ are following the law of multiple proportions.

Basically, when you have two elements, they can combine in different ways to create different compounds. In NO, it's a one to one relationship, but in NO₂ it's one nitrogen for every two oxygens. So these are the steps you'd have to take in order to determine if two compounds are following the law of multiple proportions.

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.