Question

Two oxides of a metal contain 50% and 40% of the metal, respectively. The formula of the first oxide is MO. Then, the formula of the second oxide is :

• A. $M{O}_2$
• B. $M_2{O}_3$
• C. $M_{2}O$
• D. $M_2{O}_5$

Answer 1

The correct option is B $M_2{O}_3$

For first oxide, $M=50$% ,$O=50$%.
Molecular formula of first oxide is $MO$ and its molecular mass is $16+x$ where $x$ is the atomic mass of $M$.

$\therefore$$\left(\frac{x}{16+x}\right)\times 100=50$
$\frac{x}{16+x}=\frac{1}{2}$
$2x=16+x$
$x=16$
Let us determine formula of second oxide

	$M$	$O$
mass per 100 g	40	60
moles	$\frac{40}{60}$	$\frac{60}{16}$
Divided by 2.5	$1$	$1.5$
Converting into simple whole number	2	3

Molecular formula of second oxide is $M_2{O}_3$