# 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?

May 31, 2016

$C u S {O}_{4} \cdot 5 {H}_{2} O$

#### Explanation:

We ASSUME that there are $100 \cdot g$ of copper salt, and work out the molar quantities:

$C u :$ $\frac{25.5 \cdot g}{63.55 \cdot g \cdot m o {l}^{-} 1}$ $=$ $0.401 \cdot m o l$ $C u$.

$S :$ $\frac{12.9 \cdot g}{32.06 \cdot g \cdot m o {l}^{-} 1}$ $=$ $0.401 \cdot m o l$ $S$.

$O :$ $\frac{25.6 \cdot g}{15.999 \cdot g \cdot m o {l}^{-} 1}$ $=$ $1.60 \cdot m o l$ $O$

${H}_{2} O :$ $\frac{36.0 \cdot g}{18.01 \cdot g \cdot m o {l}^{-} 1}$ $=$ $2.00 \cdot m o l$ ${H}_{2} O$

To get the empirical formula, we divide thru by the LOWEST molar quantity, i.e. $0.401 \cdot m o l$, to give:

$C u S {O}_{4} \cdot 5 {H}_{2} O$, $\text{copper sulfate pentahydrate}$

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?