1. Assume a 100g sample:

* This makes the calculation easier.

* We have 76.9 g of CaSO3 and 23.1 g of H2O.

2. Convert grams to moles:

* CaSO3: (76.9 g CaSO3) / (120.14 g/mol CaSO3) = 0.640 moles CaSO3

* H2O: (23.1 g H2O) / (18.015 g/mol H2O) = 1.28 moles H2O

3. Find the mole ratio:

* Divide the number of moles of each component by the smallest number of moles (0.640 moles CaSO3 in this case).

* CaSO3: 0.640 moles / 0.640 moles = 1

* H2O: 1.28 moles / 0.640 moles = 2

4. Write the formula:

The mole ratio tells us that for every 1 mole of CaSO3, there are 2 moles of water.

Therefore, the formula for the hydrate is CaSO3 · 2H2O.