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

Apr 13, 2017

How would you show that it is the $\text{heptahydrate}$?

#### Explanation:

Possibly, the best means would be by complexometry.

So, perhaps you would take thesalt, $Z n S {O}_{4} \cdot 7 {H}_{2} O$, or one of its several hydrates, and perform a titration with $\text{EDTA}$.................

And $\text{EDTA}$ reacts with $Z {n}^{2 +}$ with 1:1 stoichiometry. With standardized $\text{EDTA}$, and an accurate mass of the hydrate, you could establish the degree of hydration of the zinc salt.

Apr 13, 2017

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 $Z n S {O}_{4}$, 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 $Z n S {O}_{4} \cdot x {H}_{2} O$

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: $Z n S {O}_{4} \to Z n O + S {O}_{3} \left(g\right)$ and you will get a value of $x$ that is too high.