Question

Element X has two isotopes. The table gives information about these isotopes.

X-63: Has an atomic mass of 62.9296 amu and an abundance of 69.15%
X-64: Has an atomic mass of 64.9278 amu and an abundance of 30.85%

The average atomic mass of element X is ____
amu.

Answer 1

`A_r = 62.9296 \; \text{amu} * 69.15 \% + 64.9278 \; \text{amu} * 30.85 \%\\\phantom{A_r} = 63.5460 \; \text{amu}`

The idea is to sum up the product of atomic mass and abundance for each of the isotope- e.g. 62.9296 and 69.15% for X-63- to find the average of isotope atomic mass weighted regarding their abundance, which is by definition the relative atomic mass of the element.

Answer 2

Answer : The average atomic mass of an element X is, 63.546 amu

Solution : Given,

Mass of isotope X-63 = 62.9296 amu

% abundance of isotope X-63 = 69.15% = 0.6915

Mass of isotope X-64 = 64.9278 amu

% abundance of isotope X-64 = 30.85% = 0.3085

Formula used for average atomic mass of an element X :

`\text{ Average atomic mass of an element}=\sum(\text{atomic mass of an isotopes}* {{\text { fractional abundance}})`

`\text{ Average atomic mass of an element X}=\sum[(62.9296*0.6915)+(64.9278* 0.3085)]`

`\text{ Average atomic mass of an element X}=63.546amu`

Therefore, the average atomic mass of an element X is, 63.546 amu