Law of definite proportion states that a chemical compound always contains a fixed proportion of elements by mass. For example water molecule has two hydrogen atom and one oxygen atom. The ratio between hydrogen and oxygen will always remain 2:1 and it will never change. If we change the ratio of hydrogen and oxygen by 3:1, then hydronium ion is obtain and we cannot consider it as water molecule. The proportion between carbon and oxygen in carbon dioxide is always 1:2 and in carbon mono oxide is 1:1.