Element Y has two naturally occurring isotopes: isotope A with mass 40 u and abundance 60 %, and isotope B with mass 41 u and abundance 40 %. Calculate the average atomic mass of element Y.

medium 2 marks Class 11 · Chemistry · Some Basic Concepts of Chemistry

Answer

40.4 u

The correct answer is 40.4 u — Compute contribution: $40 \times 0.60 = 24.0$ and $41 \times 0.40 = 16.4$ .

Step-by-step solution

Use weighted average: $\overset{m}{ˉ} = m_{1} p_{1} + m_{2} p_{2}$ where $p$ is fractional abundance.
Convert percentages to fractions: 60 % = 0.60 and 40 % = 0.40.
Compute contribution: $40 \times 0.60 = 24.0$ and $41 \times 0.40 = 16.4$ .

Common mistake: Using percent directly instead of converting to fraction

Concept tested: Average atomic mass of isotopes