Question

The following data is given to find the formula of a Hydrate:
a. Mass of crucible: 13.56g
b. Mass of crucible and salt CaSO4 .H2O hydrate: 16.05g
c. Mass of crucible and salt CaSO4 anhydrate: 15.07
Find the mass of CaSO4.H2O hydrate, the mass of CaSO4 anhydrate, the mass of water,
the percent of water, the mole of water, the mole of salt (anhydrate), and the ratio of mole
of water to mole of salt (anhydrate).

corrected! the formatting was messed up when I first posted.

Answer 1

The masses can be found by substractions:

• Mass of CaSO₄.H2O (hydrate):

16.05 g - 13.56 g = 2.49 g

• Mass of CaSO₄ anhydrate:

15.07 g - 13.56 g = 1.51 g

• The mass of water is equal to the difference between the mass of the hydrate and the mass of the anhydrate:

2.49 g - 1.51 g = 0.98 g

• The percent of water is found by the formula:

massWater ÷ massHydrate * 100%

0.98 g ÷ 2.49 g * 100% = 39.36%

• The mole of water is calculated using water's molecular weight (18g/mol):

0.98 g ÷ 18 g/mol = 0.054 mol water

• A similar procedure is made for the mole of salt (CaSO₄ = 136.14 g/mol)

1.51 g ÷ 136.14 g/mol = 0.011 mol CaSO₄

• The ratio of mole of water to mole of anhydrate is:

0.054 mol water / 0.011 mol CaSO₄ = 0.49

In other words the molecular formula for the hydrate salt is CaSO₄·0.5H₂O