Question

Atomic weight of $Ne$ is 20.2. $Ne$is a mixture of $Ne^{20}$and$Ne^{22}$. Relative abundance of heavier isotope is

• A. 90
• B. 20
• C. 40
• D. 10

Answer 1

Answer: (D)

10

Answer 2

Average atomic weight/ The average isotopic weight

& \text{\% of 1st isotope } \times \text{ relative mass of 1st isotope } + \\ & \text{\% of 2nd isotope } \times \text{ relative mass of 2nd isotope} \end{aligned}}{\text{100}}$$ $\therefore\mspace{6mu}\mspace{6mu} 20.2 = \frac{a \times 20 + (100 - a) \times 22}{100}$; $\therefore\mspace{6mu}\mspace{6mu} a = 90$; per cent of heavier isotope $= 100 - 90 = 10$