Question

Suppose two elements $X$ and $Y$ combine to form two compounds $X{Y}_2$ and $X_3{Y}_2$. $0.1$ mol of former weighs $10$ g while $0.05$ mol of the latter weighs $9$ g. The atomic masses of $X$ and $Y$ are, respectively:

• A. $60$ and $40$
• B. $30$ and $40$
• C. $40$ and $30$
• D. $40$ and $60$

Answer 1

Answer: (C)

$40$ and $30$

The molecular weight of $X{Y}_2=\frac{10}{0.1}=100$ g/mol.

The molecular weight of $X_3{Y}_2=\frac{9}{0.05}=180$ g/mol.

Let $a$ and $b$ represent the atomic masses of $X$ and $Y$ respectively.

The molecular weight of $X{Y}_2=a+2b=100$ g/mol.

The molecular weight of $X_3{Y}_2=3a+2b=180$ g/mol.

On solving these two equations, we get $a=40$ and $b=30$.