A 11.294 g sample of an unknown hydrate of cobalt(II) bromide is heated until all the water of hydration is removed. The CoBr2 that remains has a mass of 8.0000 g.

Question

How many moles of CoBr₂ are in the sample? How many grams of water were lost in the dehydration? How many moles of water were lost? What is the value of "n" in the formula CoBr2 · n H₂O? (1, 2, 3, 4...). 

Answer 1

We start with a sample weighing 11.294 g. This sample has some water (H2O) attached to it (a hydrate). We then remove the water by heating the sample, and the sample now weighs 8.0000 g. So, it is simple to determine the mass of water that was lost (present)....

mass of H₂O = 11.294 g - 8.0000 g = 3.294 g H₂O

Using the molar mass of H₂O (18.015 g / mole), we can now calculate moles of H₂O lost...

moles H₂O = 3.294 g H₂O x 1 mol / 18.015 g = 0.1828 moles H₂O

To find the moles of CoBr₂, we will use the mass of CoBr₂ (8.000 g) and the molar mass (218.7 g / mol)...

moles CoBr₂ = 8.000 g x 1 mol / 218.7 g = 0.03658 moles CoBr₂

Finally, to find the value of n in CoBr₂ • nH₂O, we use the mole ratio of H₂O and CoBr₂ ....

n = 0.1828 mols H₂O / 0.03658 mols CoBr₂ = 4.997 so n = 5 mols H₂O per 1 mole CoBr₂ = CoBr₂•5H₂O