Question

Suppose the elements $X$ and $Y$ combine to form two compounds $X Y_{2}$ and $X_{3} Y_{2} .$ When 0.1 mole of $X Y_{2}$ weighs $10\, g$ and $0.05\, mole$ of $X _{3} Y _{2}$ weighs $9 \,g ,$ the atomic weights of $X$ and $Y$ are

• A. 40,30
• B. 60,40
• C. 20,30
• D. 30,20

Answer 1

Answer: (A)

Let atomic weight of element $X$ be $x$ and that of element Y be y.

For $X Y_{2}, n=\frac{w}{M o l . w t}$

$0.1=\frac{10}{x+2 y}$

$\Rightarrow x+2 y=\frac{10}{0.1}=100 \dots$ (i)

For $X_{3} Y_{2}, n=\frac{w}{\text { Mol. wt. }}$

$0.05=\frac{9}{3 x+2 y}$

$ \Rightarrow 3 x+2 y=\frac{9}{0.05}=180\dots$ (ii)

On solving equations (i) and (ii), we get $y=30$

$x+2(30)=100$

$\Rightarrow x=100-60=40$