What is the difference between The Law of Constant Proportion and The Law of Definite Proportion????

Dear student,
Law of constant proportion is also known as law of definite proportion. They mean the same thing.
Law of Constant Proportion : This law explains about composition of components in chemical compounds, hence it is also called as law of constant composition. According to this law, " A chemical compound is always formed by same element combined together in a fixed ratio by mass."

 By this law, we can say that, when we tale different samples of a single compound from different areas and location, we will find that each sample has fixed composition of constituent elements by mass, i.e , the elements will have similar %age in every sample.

So, this law may also be explained as; every compound exists with same percentage or proportion of constituent elements in every time ,everywhere.

For example, we take a sample of water H₂O, we see that in every 18gm of water , hydrogen is 2gm and oxygen is 16gm by mass (as molecular mass of H₂O= 2x1 +16= 18).

Hence the ratio by mass of hydrogen to oxygen in  H₂O molecule  will be, 2;16 = 1:8

We, find that in each sample of water the ratio will be same.

Regards