A silicon -containing ore has 92% 28Si, 5% 29Si and 3% 30Si. Calculate the relative atomic mass of the silicon.

Analyze the question: You're given the isotopes of silicon and their percentage abundances. The relative atomic mass is the weighted average of the isotopic masses.

Concept:

Relative atomic mass = Σ (isotopic mass × % abundance) ÷ 100

Solution:

• ²⁸Si: 28 × 92 = 2576
• ²⁹Si: 29 × 5 = 145
• ³⁰Si: 30 × 3 = 90

Total = 2576 + 145 + 90 = 2811

Relative atomic mass = 2811 ÷ 100 = 28.11

Check: The answer should be close to 28, since most of the sample (92%) is ²⁸Si, and slightly above it because of the heavier isotopes. 28.11 fits this.

Answer: C. 28.11

Tip: Start from the lightest isotope and add only the "extra" mass: 28 + (5 × 1 + 3 × 2)/100 = 28 + 11/100 = 28.11. This saves you from multiplying large numbers in the exam hall.