An element have two isotopes with mass number 16 and 18.The average atomic weight is 16.5.The percentage abundance of these isotopes are _______and____________respectively.

The correct option is A

75,25

Assume % abundance of the element with atomic number16 is 'a' then % abundance of the element with atomic number18 is 100-'a' in nature.

The average atomic mass of element "X" will be$\left[16\times \frac{\text{'}\mathrm{a}\text{'}}{100}\right]+\left[18\times \frac{\text{'}100-\mathrm{a}\text{'}}{100}\right]=16.5$.

$\left[16\times \text{'}\mathrm{a}\text{'}\right]+\left[18\times (\text{'}100-\mathrm{a}\text{'})\right]=1650$

$\left[16\times \text{'}\mathrm{a}\text{'}\right]+\left[1800-18\times \mathrm{a}\text{'}\right]=1650$

$16\times \text{'}\mathrm{a}\text{'}-18\times \mathrm{a}\text{'}=1650-1800$

$2\times \text{'}\mathrm{a}\text{'}=150$

$\text{'}\mathrm{a}\text{'}=\frac{150}{2}=75$

% abundance of the element with atomic number16 is '75' then % abundance of the element with atomic number18 is 100-'75'=25 in nature.

Conclusion Statement: Option (a) is the correct answer.