A hydrate of Na2CO3 contains 276.0g. It was heated strongly to drive off the water. The mass of the anhydrous Na2CO3 was found to be 102.0g.Find...

First determine the number of moles of Na2CO3 after heating:

Formula weight of Na2CO3=

2 Na - 2x22.99

1 C  - 12.01

3O  - 3x15.99

Na2CO3=105.96

Moles Na2CO3 = mass after heating / formula weight

= 102.0g/105.96 = 0.96 moles Na2CO3

The weight change from heating is a result of water loss, so the weight of water in the hydrate was initial weight minus final weight:

276.0g - 102.0g = 174.0g

The molecular weight of water is 18.01g/mole, so the number of moles of water is weight in hydrate from water divided by its molecular weight:

174.0g/18.01g = 9.67 moles water

Divide this by the number of moles of Na2CO3 to find the number of moles of water per mole of Na2CO3.

9.67/9.6 = 10

So the empirical formula is Na2CO3 x 10H2O