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.