Question

A 6.2351 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 5.0000 g.

1. How many moles of CoBr2 are in the sample? mol

2. How many grams of water were lost in the dehydration? g

3. How many moles of water were lost? (mol)

4. What is the value of "n" in the formula CoBr2 · n H2O? (1, 2, 3, 4...).

Answer 1

Molar mass of cobalt(ii) bromide hexahydrate = 326.83 g/mol

Molar mass of cobalt(ii) bromide = 218.74 g/ mol

Mass of of cobalt(ii) bromide with hydrate = 6.2351 g

Mass of water = 6.2351 g - 5.000 g = 1.2351 g

Moles of water = 1.2351 g / 18 g/mol = 0.0686 Mol

Mass of cobalt(ii) bromide = 5.000 gm

Moles of cobalt(ii) bromide = 5.000 gm / 218.74 g/mol = 0.02285 Moles

Ratio of cobalt(ii) bromide and water =  0.02285 M :  0.0686 M = 1 : 3

using the above calculation we can answer all of the given questions

1. How many moles of CoBr2 are in the sample? 0.00285 Moles

2. How many grams of water were lost in the dehydration? 1.2351 gm

3. How many moles of water were lost? 0.0686 Moles

4. What is the value of "n" in the formula CoBr2 · 3 H2O