Let's examine the law of definite proportions. In 1799, the French chemist Joseph L. Proust originated the law of definite proportions. Because of its immense contributions to it, it's sometimes referred to simply as Proust's law. But it also goes by the law of constant or definite composition. The law itself uses what we call mass ratios. These mass ratios are fractions or proportions of elements by mass. The whole concept behind the law of definite proportions is that different samples of a pure chemical compound always contain the same proportions of elements by mass. So let's say, for example, that I take a sample from New York City, and I suspect that it's CO_{2}. And I take a sample in another city, London, England, and I suspect that's CO_{2}. If we're following the law of different proportions, both samples should have the same mass ratio. Now for the mass ratio, we're going to place the element with the larger mass on top.

Now, going back to our whole idea of CO_{2} in 2 different cities. Remember, CO_{2} itself is composed of 1 carbon and 2 oxygens. And we know that if we look at the periodic table, the atomic mass of carbon is 12.01 grams per mole, and the mass of oxygen is around 16 grams per mole. When you multiply the number of each element by its atomic mass, we'll see how much of it contributes to the overall mass of CO_{2}. So carbon itself contributes 12.01 grams total, and oxygen contributes 32 grams total. Now using the mass ratio, we place the larger mass on top, which is the 32 grams of oxygen, and the smaller mass on the bottom. When we divide those two numbers, it gives us approximately 2.66. What that 2.66 is telling us is that we have 2.66 oxygens for every one carbon. This would be our mass ratio. And if I were examining 2 samples of CO_{2}, 1 from New York City and 1 from London, if they both were indeed CO_{2}, they both should give me back the same exact mass ratio. That's what the law of definite proportion hinges on. If we know the mass ratio of a given sample, and we're examining it against an unknown sample, I use this law to determine if they're the same sample. Alright. Now we get the idea behind the law of definite proportions, so let's move on. Let's talk about calculations and different types of problems associated with the law of definite proportions.