Question

Magnesium has three naturally occurring isotopes having atomic masses 24 u, 25 u, and 26 u which occur in the ratio of 8:1:1 respectively. Find out the average atomic weight of magnesium.

• A.
  25.4 u
  
• B.
  24.6 u
  
• C.
  24.3 u
  
• D.
  26.1 u
  

Answer 1

The correct option is C

24.3 u

Average atomic weight is the weighted average of the atomic weights of all the natural isotopes of an element.
The weighted average is calculated by adding the products of the atomic weights of the isotopes with their respective ratios and dividing the sum total by the sum of the ratios.
Average atomic weight of Mg $=\frac{(24\times 8)+(25\times 1)+(26\times 1)}{8+1+1}u$
$=\frac{192+25+26}{10}=\frac{243}{10}=24.3u$
Hence, the average atomic weight of Mg will be 24.3 u.