Right option is (c) 40
To explain I would say: Given,
Concentration, c = 0.3 mole/L
Volume, v = 100 ml = 0.1 L
Number of moles, n = concentration (mole/L) x volume (L)
n = 0.3 mole/L x 0.1 L = 0.03 moles
0.03 moles of XCl2 corresponds to 3.33g, then 1 mole contains 3.33/0.03 = 111 g of XCl2.
Hence, molecular mass = 111 g/mole.
Let x be the atomic mass. Then –
111 = x + 2(35.5)
x = 40g/mole.