Suppose the elements $x$and $y$ combine to form two compounds ${\mathrm{xy}}_2$and ${\mathrm{x}}_3{\mathrm{y}}_2$. When 0.1 mole of ${\mathrm{xy}}_2$ weighs 10 g and 0.05 mole of ${\mathrm{x}}_3{\mathrm{y}}_2$ weighs 9 g, the atomic weights of X and Y are

Let at. wt. of X = a
At. wt. of Y = b
Given, 0.1 mol of XY2 = 10 g
1 mol ..........= ?
Molecular wt $=\frac{1}{0.1}\times 10=100$
Now, a + 2b = 100 ---(1)
3a + 2b = 180 ----(2)
on solving (1) and (2),
a = 40, b = 30.