Question

Suppose elements $X$ and $Y$ combine to form two compounds, $X{Y}_2$ and $X_3{Y}_2$, where $0.1$ mol of former weigh $10$ g while $0.05$ mol of the latter weigh $9$ g. What are the atomic weights of $X$ and $Y$ respectively?

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

Answer 1

The correct option is A $40,30$

I. $X+2Y\to X{Y}_2$

$0.1$ mol of $X{Y}_2=10$ g

$1$ mol of $X{Y}_2=100$ g

II. $3X+2Y\to X_3{Y}_2$

$0.05$ mol of $X_3{Y}_2=9$ g

$1$ mol of $X_3{Y}_2=\frac{9}{0.05}=180$ g

$\therefore X+2Y=100...\left(i\right)$ (Molecular weight of $X{Y}_2=100$ g/mol)

$3X+2Y=180...\left(ii\right)$ (Molecular weight of $X_2{Y}_3=180$ g/mol)

Solving $\left(i\right)$ and $\left(ii\right)$ for $X$ and $Y$, we get

$X=40$ g

$Y=30$ g