 Let's talk about law of definite proportions or it's sometime called law of constant proportions. Now this law states that in any chemical substance the elements are always present in definite proportions by mass. Now instead of proportions, we can call them ratio to make the definition simpler. So we can say that the elements are always present in definite or constant ratio by mass. Let's try to understand this with the example of water. So we know that water is H2O and if we apply this rule to water, it says that water as a compound is always formed from the same ratio of hydrogen and oxygen. And specifically we can see that water is formed from two atoms of hydrogen. So this is two atoms of hydrogen and one atom of oxygen. So this will always be the ratio of hydrogen to oxygen atoms in any sample of water. It doesn't matter if it is tap water or water in ocean or a lake. The ratio of hydrogen and oxygen atoms in that sample of water will be 2 is to 1. Now this rule also extends to mass. So we know that every atom has a mass of its own. And we know that one atom of hydrogen weighs one atomic mass unit. So that will be two atoms will be two atomic mass units. So let me write 2AMU. One atom of oxygen has an atomic mass of 16AMU. So that will be 16AMU, 16 atomic mass units. And now from here I can say that the mass ratio of oxygen to hydrogen will be 16 is to 2. And this can be simplified to 8 is to 1. This is a ratio of oxygen to hydrogen by mass. And what this really means is that in any sample of water, either a drop or water in a glass or water in an ocean, doesn't matter if you take 50 litre, 10,000 litres or even just one ml of water. The mass of oxygen present in that sample will be 8 times the mass of hydrogen that is present in that sample. And this is the law of definite proportions. It holds true for every other chemical substance, even for carbon dioxide. So you see carbon dioxide being formed when organic matter burns. You also exhale carbon dioxide. And doesn't matter how much carbon dioxide you take, the ratio of oxygen to carbon atoms or the mass of oxygen to carbon will always be the same in any sample of carbon dioxide.