Question

The three stable isotopes of neon ${ }_{10} Ne ^{20},{ }_{10} Ne ^{21}$ and ${ }_{10} Ne ^{22}$ have respective abundances of $90.51 \%, 0.27 \%$ and $9.22 \%$. The atomic masses of the three isotopes are $19.99\, u$, $20.99 \,u$ and $21.99\, u$ respectively. The average atomic mass of neon is

• A. 11.18 u
• B. 15.18 u
• C. 20.18 u
• D. 10.18 u

Answer 1

Answer: (C)

The masses of three isotopes are $19.99$ u, $20.99$ u, $21.99 u$
Their relative abundances are $90.51 \%, 0.27 \%$ and $9.22 \%$
$\therefore \quad$ Average atomic mass of neon is
$m=\frac{90.51 \times 19.99+0.27 \times 20.99+9.22 \times 21.99}{(90.51+0.27+9.22)}$
$=\frac{1809.29+5.67+202.75}{100}=\frac{2017.7}{100}=20.18 \,u$