Which law states, that in a chemical compound, elements always combine in a fixed proportion?

If 100 g of calcium carbonate on heating produces 44g of carbon dioxide, how much quicklime will be formed? Which law is followed for solving this problem?

20g silver nitrate solution is added to 20g of sodium chloride solution. What change in mass do you expect after the reaction and why?

In a given sample of ammonia, 9g hydrogen and 42g nitrogen are present. In another sample, 5g hydrogen is present along with nitrogen. Calculate the amount of nitrogen in the second sample.

Why definition of an atom given by Dalton is no longer valid?