Question

Suppose that A and B form the compounds $B_2{A}_3$ and $B_{2}A$. If 0.05 mole of $B_2{A}_3$ weighs 9 g and 0.1 mole of $B_{2}A$ weighs 10 g, the atomic weight of A and B respectively are:

• A. 30 and 40
• B. 40 and 30
• C. 20 and 5
• D. 15 and 20

Answer 1

The correct option is B 40 and 30

$moles=\frac{mass}{molarmass}$

then, $molarmass=\frac{mass}{moles}$

$B_2{A}_3$ molar mass $=90.05=180$

relative formula mass of $B_2{A}_3=180$

that means,

(atomic mass of $A$) $\times 3+$ (atomic mass of $B$) $2=180$

$3A+2B=180$.....(i)

$B_{2}A$ molar mass $=\frac{10}{0.1}=100u$

$2B+A=100$.....(ii)

from equation (i) and (ii)

$A=40$

$B=30$

Hence, the correct option is $B$