After 0.6523 g of is heated, the residue has a mass of 0.3423 g. Calculate the % H2O in the hydrate.

What was the actual number of moles of water per formula unit CoCl2?

1. How many grams have been lost?

2. Divide that by the initial mass. That's the fraction of H2O (you assume that only water was lost. There is a chemical basis for this assumption).

3. You know the mass of CoCl2 that is left. Calculate the number of moles (formula weight) of this. There is one mole of Co per mole of CoCl2.

4. You also know how the weight of water that was lost. Calculate the number of moles of this.

From #3 and #4, you can ratio the number of moles ot water to the number of moles of Co. That's the actual number of moles of water per CoCo2.