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 Xhydrate. You must determine the number for X.

Answer 1

0.04896 moles of water

Step-by-step explanation:

The mass of the test tube is 10.3362 g

The mass of the test tube + hydrated calcium sulfate is 14.5549 g

The mass of the test tube + calcium sulfate is 13.6737 g

Therefore substracting these you can get the mass of hydrated calcium sulfate which is:

14.5549 - 10.3362 = 4.2187 g

The mass of calcium sulfate:

13.6737 - 10.3362 = 3.3375 g

And finally the mass of water in the hydrated compound:

4.2187 - 3.3375 = 0.8812 g

The molar mass of water is 18 g/mol, then the number of moles is:

moles of water = 0.8812 / 18 = 0.04896 moles

Now to find the X in calcium sulphate Xhydrate

You have to find the proportion in moles between calcium sulphate and water.

The mass of calcium sulphate is 3.3375 g and the molar mass is 136.14 g/mol. With that you calculate the moles:

moles of calcium sulphate = 3.3375 / 136.14 = 0.0245

To get the proportion you have to divide by the least amount, in this case 0.0245

water = 0.04896 / 0.0245 = 2

calcium sulphate = 0.0245 / 0.0245 = 1

Therefore the formula is CaSO₄ · 2 H₂O, X = 2