Question

HYDRATED CALCIUM SALT CONTAINS 21% OF WATER OF CRYSTALLISATION. CALCULATE THE NUMBER OF MOLECULES OF WATER OF CRYSTALLISATION THAT IS X IN THE HYDRATED COMPOUND.

Answer 1

Answer:- The value of X is 2.

Solution:- Let's say we have 100 g of the hydrate. There is 21% water in it. 21% water means there are 21 g of water in 100 g of the hydrate.

So, mass of anhydrous salt = 100g - 21g = 79g

Let's convert the grams of anhydrous salt and water to moles as:

moles of water = 21g H2O ( 1 mol/18g)

moles of water = 1.17 mol

moles of CasO4 = 79g CasO4 (1 mol /136.14 g)

moles of = 0.58 mol

Now, let's calculate the ratio of their moles and for this we divide the moles of each by the least one of them. Here the least one is salt. So:

moles of water = 1.17/ 0.58 = 2

moles of salt = 0.58/0.58 = 1

The ratio of salt and water moles is 1:2. It means the value of X is 2 and the formula of the hydrate is CuSo4.2H2O

Hope this helps !!!