How is water of hydration related to measurement and how it is related to the law of definite composition?

Answer 1

I don't know how it is related to measurement; I don't understand the question. For the second part, water of hydration is a definite proportion each time and every time (although there are hydrates that have different moles of hydration. For example, CuSO4 usually is CuSO4.5H2O but that water can be driven off in stages and it may also be CuSO4.H2O.