Question

You placed a sample of a hydrate of calcium chloride (CaCl₂) in a weighed test tube, and weighed the filled test tube.
Then you heated it until no more water was evolved. After cooling, you weighed the test tube again.

Mass of empty tube (g) 13.5
Mass of filled tube before heating (g) 18.5
Mass after cooling (g) 16.2

Calculate the following:
1.Mass of hydrate originally taken  

2.Mass of water evolved

3.Mass of anhydrous CaCl2 formed

4.Moles of anhydrous CaCl2

5.Moles of water evolved

6.Moles of water in the molecular formula of the hydrate (give your answer to 2 decimal places)

Answer 1

1)Mass of hyrate = Before heating - empty tube

= 18.5- 13.5

= 5 g.

2) Mass of water evolve = Before heating - After cooling

= 18.5-16.2

=2.3 g

3) mass of anhydrous CaCl2 = mass of hydrate - mass of water

   = 5 - 2.3

   = 2.7 g

4) moles of anhydrous CaCl₂ = wt / m.wt molecular weight is 110.98

= 2.7 / 110.98

= 0.0243

5) moles of water evolved = 2.3/18 water molecular weight = 18

= 0.1277

6) moles of hydrate = wt/ m.wt

   = 5 / 110.98

   =0.045

moles of water from molecular formula= moles of hydrated - moles of anhydrous CaCl2

   =0.045 - 0.0243

   = 0.0207