Question

How could you establish the formula of `ZnSO_4*7H_2O`?

Answer 1

How would you show that it is the `"heptahydrate"`?

Explanation:

Possibly, the best means would be by complexometry.

So, perhaps you would take thesalt, `ZnSO_4*7H_2O`, or one of its several hydrates, and perform a titration with `"EDTA"`.................

And `"EDTA"` reacts with `Zn^(2+)` with 1:1 stoichiometry. With standardized `"EDTA"`, and an accurate mass of the hydrate, you could establish the degree of hydration of the zinc salt.

Answer 2

You could use heat to remove the cristal water.

Explanation:

If you weigh an amount of hydrate and heat it (not too hot!) untill the weigh stays constant, then:

End weight represents anhydrous `ZnSO_4`, which you can convert to mols.

Weight loss represents the cristal water removed, which you also convert to mols.

The ratio of the two gives you the `x` in `ZnSO_4*xH_2O`

Note:
There are three ways this can go wrong:
(1) If you don't heat long enough, not all the water will be gone, and you'll end up with a lower number for `x`
(2) If the sample of hydrated salt is already partly dehydrated, you'll also get an `x` that is too low.
(3) If you heat too long, or too hot, part of the zinc sulphate may disintegrate: `ZnSO_4->ZnO+SO_3(g)` and you will get a value of `x` that is too high.