Question

Barium hydroxide forms a hydrate, Ba(OH)2 · xH2O. What is the value of x if the compound is 45.7% by mass water?

Answer 1

Let us assume weight of hydrate to be 100g.

Then weight of water in it is 45.7g and remaining is barium hydroxide =54.3 g

molar mass of water is 18 and molar mass of Ba(OH)2 is 171.

The number of moles of each in the hydrate is

moles of water = 45.7/18 =2.538 and moles of barium hydroxide = 54.3/171 =0.3175

In the hydrate the ratio of moes of water to barium hydroxide is a simple fixed ratio. thus the ratio is 2.538/0.315= 7.998 = 8.0

Thus for one mole of barium hydroxide , there are 8 moles of water molecules and thus the hydrate is Ba(OH)₂ . 8H2O.