Question

A 1.000 g sample of an unknown hydrate of cobalt chloride is gently dehydrated. The resulting mass is 0.546 g. The cobalt is isolated and weighs 0.248 g. What is the formula of the hydrate? Make sure you use correct number of significant figures, or your answer could be incorrect. Show your work on all calculations.

Answer 1

Answer:

Explanation:

From the 1 g sample you have:

0.546 grams of cobalt chloride

1-0.546=0.454 grams of water

Now:

1) The salt

Of the 0.546 g, 0.248 g are cobalt (Mr=58.9) and the rest id Cl (Mr=35.45):

$n_{Co}=\frac{0.248 g}{58.9 g/mol}=4.21*10^{-3}mol$

$n_{Cl}=\frac{0.298 g}{35.45g/mol}=8.4*10^{-3}mol$

Dividing:

$\frac{n_{Cl}}{n_{Co}}=\frac{8.4*10^{-3}mol}{4.21*10^{-3}mol}=2$

So the molecular formula will be:

$CoCl_2$

2) The water

The water's molecular weight is M=18 :

$n_{w}=\frac{0.454g}{18g/mol}=0.025 mol$

Bonding with the Co:

$\frac{n_{w}}{n_{Co}}=\frac{0.025mol}{4.21*10^{-3}mol}=6$

The complete formula of the hydrate:

$CoCl_2*6 H_2O$