Question

An element has three stable isotopes. One has a mass number of 63 and an abundance of 50%. The second has a mass number of 65 and an abundance of 30%. The third has a mass number of 67 and an abundance of 20%. Calculate the relative atomic mass of this element.

Answer 1

It will be
(63×50+65×30+67×20)/100

Answer 2

The relative atomic mass of element having three isotopes with mass number 63, 65 and 67 is  64.40 amu.

Given,

Percentage(percentage abundance) of element with mass number 63=50%

Percentage(percentage abundance) of element with mass number 65=30%

Percentage(percentage abundance) of element with mass number 67=20%.

To find,

the relative atomic mass of the element.

Solution:

• The relative atomic mass or average atomic mass of an element is given by the sum of the masses of isotope, each multiplied by its relative abundance percentage.
• $AAM=\frac{M1X(abundance)1+M2X(abundance)2+)3}{100}$.
• Its unit can be amu(atomic mass unit) or grams.

The relative atomic mass of the element will be,

$AAM=\frac{M1X(abundance)1+M2X(abundance)2+M3(abundance)3}{100}\\AAM=\frac{63X50+65X30+67X20}{100} \\AAM=\frac{3150+1950+1340}{100} \\AAM=\frac{6440}{100} \\AAM=64.40 amu.$

Hence, the relative atomic mass is 64.40 amu.

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