Asample of hydrated cobalt (ii) chloride has a mass of 5.22 g. after heating, it has a mass of 2.85 g. what is the percent by mass of water in the hydrate?

mass of water in hydrate = 2.37 grams

Explanation:
The mass of the hydrated cobalt (III) chloride is the summation of the salt and the water it contains.
This means that:
Total mass of sample = mass of salt + mass of water

Now, we are given that:
total mass of sample = 5.22 grams
mass of salt = mass of sample after heating = 2.85 grams

Substitute to get the mass of water as follows:
5.22 = mass of water in hydrate + 2.85
mass of water in hydrate = 5.22 - 2.85
mass of water in hydrate = 2.37 grams

Hope this helps :)