Calculate the formula for the following hydrate composed of 76.9% CaSO3 and 23.1% H2O.

Assume you have 100 g of the hydrate: that gives 76.9 g CaSO₃ and 23.1 g H₂O. Their amounts are 76.9 ÷ 120.1 ≈ 0.640 mol CaSO₃ and 23.1 ÷ 18.0 ≈ 1.28 mol H₂O. Dividing both by 0.640 gives a ratio of 1 CaSO₃ to 2 H₂O, so the formula is CaSO₃ • 2H₂O.

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