Relative atomic mass of an element Q is 14.0037.Q has two isotopes.15^Q makes up 0.37% of all the atoms.calculate the mass number of the second isotope of element Q

The relative atomic mass (Ar) of an element is the weighted average of the mass numbers of its isotopes, taking into account their natural abundances.
For two isotopes, the formula is:

The abundances are usually expressed as decimals (percentage divided by 100).

• Relative atomic mass of Q ($\text{Ar}_{\text{Q}}$) = 14.0037
• Isotope 1: $^{15}\text{Q}$
  • Mass number of isotope 1 = 15
  • Abundance of isotope 1 = 0.37% = $\frac{0.37}{100} = 0.0037$
• Isotope 2: Let its mass number be $x$.
  • Abundance of isotope 2 = ?

Since there are only two isotopes, their abundances must sum to 100%.
Abundance of isotope 2 = 100% - Abundance of isotope 1
Abundance of isotope 2 = 100% - 0.37% = 99.63%
As a decimal, abundance of isotope 2 = $\frac{99.63}{100} = 0.9963$

Subtract 0.0555 from both sides:

Divide by 0.9963:

The mass number must be a whole number.

The mass number of the second isotope is $\text{14}$.