Ncert Exercises & Solutions

13.2 The three stable isotopes of neon: 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. Obtain the average atomic mass of neon.

Hint: The average atomic mass of neon is the weightage average of its isotopes.
Step 1: Find the average weight of the neon atom.

The atomic mass of

{}_{10}^{20}{Ne}, m_{1} = 19.99 u

The atomic mass of

{}_{10}^{20}{Ne}, n_{1} = 90.51 %

The atomic mass of

{}_{10}^{21}{Ne}, m_{2} = 20.99 u

Abundance of

{}_{10}^{21}{Ne}, n_{2} = 0.27 %

Atomic of mass

{}_{10}^{22}{Ne}, m_{3} = 21.99 u

The abundance of

{}_{10}^{22}{Ne}, n_{3} = 9.22 %

The average atomic mass of neon is given as:

m = \frac{m_{1} n_{1} + m_{2} n_{2} + m_{3} n_{3}}{n_{1} + n_{2} + n_{3}} = \frac{19.99 \times 90.51 + 20.99 \times 0.27 + 21.99 \times 9.22}{90.51 + 0.27 + 9.22} = 20.1771 u

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