100 g of an inorganic compound X. 5H2O containing a volatile impurity was kept in an oven at 150 degree celcius for 60 minutes. the weight of residue after heating is 8g. the percentage of impurity in X was?

150°C

X*5H₂O → X + 5H₂O↑

m₀=100 g

m (X) = 8 g

M (H₂O) = 18.0 g/mol

n (H₂O) = 5 mol

the mass of the water

m (H₂O) = M (H₂O) n (H₂O)

m (H₂O) = 18.0*5=90 g

the mass of the residue after removal of water

m' (X) = 100-90=10 g

the percentage of impurity in X*5H₂O

w (imp) = [m' (X) - m (X) ]/m₀

w (imp) = [10-8]/100=0.02 (2%)