Question

If $m$ atoms of element $X$ weigh 15 g and
4 $m$ atoms of element $Y$ (atomic mass = 30) weigh 45 g, then the atomic mass of the element $X$ is :

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

Answer 1

The correct option is C $40$ g

No of moles of $Y$ = $\frac{45}{30}$ = $1.5$ mol
$1.5$ mol of $Y$ = $4m$ atoms of $Y$
$1.0$ mol of $Y$ = $\frac{4m}{1.5}$ atoms of $Y$
= $\frac{8}{3}m$ atoms of $Y$

$m$ atoms of element $X$ weigh $15$ g

So, $\frac{8}{3}m$ atoms of $Y$ weigh = $\frac{15}{m}\times \frac{8m}{3}$ = $40$ g.