Element ‘A’ reacts with oxygen to form a compound $A_2{O}_3$. If 0.359 gram of a compound contains 0.128 gram of oxygen, atomic weight of A will be ____? (assuming law of constant proportions is true).

$0.359 \text{gram of }{A}_2{O}_3\text{ contain }0.128\text{ gram of oxygen }$

Let atomic mass of A is x g/mol and atomic mass of O is 16 g/mol.

Molar mass of ${\mathrm{A}}_2{\mathrm{O}}_3$ is (2a+48) g/mol.

One molecule of ${\mathrm{A}}_2{\mathrm{O}}_3$ has 2 A atoms, ratio of mass of A and ${\mathrm{A}}_2{\mathrm{O}}_3$ is $\frac{2\mathrm{a}}{2\mathrm{a}+48}.$

It is given that 0.359 gram of a compound contains 0.128 gram of oxygen, 

$\frac{2a}{2a+48}=\frac{0.359}{0.487}$

$\text{ of }{A}_2{O}_3=134.625\text{ gram of }{A}_2{O}_3$

$\text{ Total mass of ' }\mathrm{A}\text{ ' in }{A}_2{\mathrm{O}}_3=134.625-48=86.625$

$\text{ Therefore atomic mass of }A=\frac{86.625}{2}=43.3\mathrm{amu}$

Allowed value = 43 to 43.5