Question

A copper salt contains `25.5%` `Cu`, `12.9%` `S`, `25.6%` `O`, and `36.0%` `"water"`...what is the empirical formula of the hydrate?

Answer 1

`CuSO_4*5H_2O`

Explanation:

We ASSUME that there are `100*g` of copper salt, and work out the molar quantities:

`Cu:` `(25.5*g)/(63.55*g*mol^-1)` `=` `0.401*mol` `Cu`.

`S:` `(12.9*g)/(32.06*g*mol^-1)` `=` `0.401*mol` `S`.

`O:` `(25.6*g)/(15.999*g*mol^-1)` `=` `1.60*mol` `O`

`H_2O:` `(36.0*g)/(18.01*g*mol^-1)` `=` `2.00*mol` `H_2O`

To get the empirical formula, we divide thru by the LOWEST molar quantity, i.e. `0.401*mol`, to give:

`CuSO_4*5H_2O`, `"copper sulfate pentahydrate"`

I have treated the water here as I would normally treat an element; I am certainly free to do so inasmuch as we could dry the salt and measure the actual water loss.

What is the colour of this salt?