Question

You start your experiment with an empty test tube having a mass of 10.3362 g. You are working with a hydrated compound of calcium sulfate. After adding the hydrate to the empty test tube, the mass of the test tube and hydrate is 14.5549 g. After heating the hydrate, you weigh the test tube again and the mass is 13.6737 g. Determine the number of moles of water in the original hydrated calcium sulfate compound. Hint: The hydrate will be calcium sulphate ● X hydrate. You must determine the number for X.

Answer 1

Given,

Mass of test tube = 10.3362 g

Mass of test tube + hydrated compound = 14.5549 g

Mass of test tube + unhydrated compound = 13.6737 g

=> Mass of water in hydrated compound = 14.5549 - 13.6737 = 0.8812 g

Mass of unhydrated compound (CaSO4) = 13.6737 - 10.3362 = 3.3375 g

We know that,

Molar Mass of water = 18 g / mol

Molar Mass of CaSO4 = 136.14 g / mol

=> Moles of Water = 0.8812 / 18 = 0.049

and Moles of CaSO4 = 3.3375 / 136.14 = 0.0245

=> 0.0245 moles of CaSO4 contains 0.049 moles of H2O

=> 1 mole of CaSO4 contains 0.049 / 0.0245 = 2 moles of H2O

Therefore, the compound is CaSO4.2H2O

Value of X = 2