Question

The relative abundance of two isotopes of atomic weight 85 and 87 is 75% and 25% respectively. The average atomic weight of element is

• A. 86
• B. 40
• C. 85.5
• D. 75.5

Answer 1

Answer: (C)

85.5

Answer 2

Average atomic weight $=\frac{R.A.\left(1\right) ??At. mass\left(1\right) + R.A.\left(2\right) ??At. mass\left(2\right)}{R.A.\left(1\right) + R.A.\left(2\right)}$ Average atomic weight $=\frac{75\times85\times25\times87}{100}$ $=\frac{6375+2175}{100}=\frac{8550}{100}=85.50$