How to find the formula of the hydrate

Question

A student has a sample of CaSO4 hydrate and it weighs 0.4813 grams. He heats it strongly to drive off the water of hydration, and after subsequent heatings, the student finds the anhydrous compound has a constant mass of 0.3750 grams. Find the formula of the hydrate.

Answer 1

Given

Weight of hydrated CaSO₄.x H₂O = W₁ = 0.4813 g

Weight of anhydrous CaSo₄ = W₂ = 0.3750 g

Weight of H2O that was present as water of hydration is the difference between W_{1 &} W₂

W₁ - W₂ = 0.1063 g

CaSO₄.x H₂O  CaSO₄   + x H₂O 0.4812 g 0.3750 g 0.1063g

Molar Mass of anhydrous CaSO₄ = 136.14 g/mol

Molar mass of H₂O = 18.0153 g/ mol

So, convert that to moles, CaSO₄ = Given mass / molar mass

= 0.3750 / 136.14 = 0.002755 mol

Number of moles of H₂O = Given mass / molar mass

  = 0.1063 / 18.0153 = 0.0059005 mol

So,

x = 0.0059005 / 0.002755

CaSO4 . 2 H2O