Question

$H_{3}A{O}_4$ has 31.6% of "A" by mass. The atomic mass of the atom "A" is:

• A. 17
• B. 31
• C. 35.5
• D. 40

Answer 1

Answer: (B)

31

Let the atomic mass of "A" be $x$.

Given,

31.6% of $H_{3}A{O}_4=x$

$\Rightarrow \frac{31.6}{100}(1\times 3+x+16\times 4)=x$

$\Rightarrow 67+x=\frac{100}{31.6}x$

$\Rightarrow 67=\frac{68.4}{31.6}x$

$\Rightarrow x=31$

Hence, option $B$ is correct.