A sample of hydrated magnesium sulfate (MgSO4) has a mass of 12.845 g. When it is dehydrated, the anhydrous sample has a mass of 6.273 g. Which is the correct chemical formula for the hydrate?

The mass of water lost is 12.845 g − 6.273 g = 6.572 g. This is about 6.572 ÷ 18.02 = 0.365 mol of water; the anhydrous MgSO4 is about 6.273 ÷ 120.37 = 0.0521 mol. Dividing gives a water-to-MgSO4 ratio of about 0.365 ÷ 0.0521 = 7, so the formula is MgSO4•7H2O, choice B.

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