How to determine a molecular formula from this informatiion?


Would you please help me solve this example: Find the formula where the mass ratio Al:Cl=1:3.95, (The compound consists 48% of crystal water.)

If Al=27 (and Cl=35.5), there must be more atoms Cl in the formula if the mass ratio = 1:3.95. So for 27g Al (1 mole) there must be 3.95 x 27 = 106.65g Cl which is 3.004moles Cl. So the smallest ratio Al:Cl = 1:3 or AlCl3. Could be Al2Cl6? Doubt it as this exists in the vapour phase. Each mole AlCl3 weighs 27 + 35.5x3 = 133.5g

In 100g of the compound there are 48g of water, so 52g AlCl3. Moles water per 100g = 48/18 = 2.67 moles. Moles AlCl3 = 52/133.5 = 0.39moles which works out a 1 mole AlCl3:6.84 moles water. Strange figures or perhaps you can see a mistake I have made. There is a compound AlCl3.6H2O?

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