Question

How are experimental data used to show that a compound obeys the law of definite proportions?

Answer 1

Let's do this by example

Iron `Fe` and sulphur `S` react to form `FeS`

They always react in the proportion 7 `Fe` to 4 `S` (in weight units)

If you use too much iron, you can still detect the remaining iron from the (ground-up) mixture with a magnet.
If you use too much sulphur, you can still burn it, giving the characteristic `SO_2`-stench

In the past, chemists have found these ratios (or definite proportions) for almost all reactions they knew at the time.

This led to the idea of atomic weights, in which all elements were compared to the weight of hydrogen `H` as being always the smaller in the ratio-equation.

(later the unit was shifted to `C^12=12` but that's not important)

In this system `Fe=56` and `S=32` gram/mole (but the mole is a different story)