Question

Suppose the elements $X$ and $Y$ combine to form two compounds $XY_{2}$ and $X_{3}Y_{2}$ . When $0.1$ mole of $XY_{2}$ weighs $10g$ and $0.05$ mole of $X_{3}Y_{2}$ weighs $9g$ , 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 masses of $X$ and $Y$ be $A_{x}$ and $A_{Y}$ , respectively
For $XY_{2},n_{xY_{2}}=0.1=\frac{10}{ A_{X} + 2 A_{Y}}$
or $A_{x}+2A_{y}=100$
For $X_{3}Y_{2}$ ,
$n_{x_{3} r_{2}}=0.05=\frac{9}{3 A_{x} + 2 A_{Y}}$
or $3A_{x}+2A_{Y}=180$
On solving Eqs. $\left(i\right)$ and $\left(ii\right)$ , we get
$A_{x}=40gmol^{- 1}$
$A_{Y}=30gmol^{- 1}$