Question

5 g of a crystalline salt when rendered anhydrous lost 1.8 g of water. The formula weight of the anhydrous salt is 160. The number of molecules of water of crystallisation in the salt is

(a) 3

(b) 5

(c) 2

(d) 1

Answer 1

Correct option is (b) 5

Weight of water in 5 g of hydrated salt = 1.8 g

So, weight of anhydrous salt = 51.8 = 3.2g 

Weight of water associated with one formula mass (160 g) of the anhydrous salt

\(= \frac{1.8 \times 160}{3.2}\)

\(= 90 g\)

Number of moles of water of crystallization = \(\frac{90}{18} = 5\)