An element, Q, contains 69% of$^{63}$Q and 31% of $^{65}$Q. What is the relative atomic mass of Q?

Analyze the question: Q has two isotopes, ⁶³Q (69%) and ⁶⁵Q (31%). The relative atomic mass is the weighted average of the isotopic masses.

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

Solution:

• Contribution from ⁶³Q: 63 × 69 = 4347
• Contribution from ⁶⁵Q: 65 × 31 = 2015

Sum = 4347 + 2015 = 6362

Relative atomic mass = 6362 ÷ 100 = 63.62 ≈ 63.6

Check: The answer must lie between 63 and 65. Since most atoms (69%) are ⁶³Q, it should be closer to 63. 63.6 fits this.

Tip: A quick shortcut is to start from the lighter isotope and add the excess weighted by abundance: 63 + (2 × 0.31) = 63.62. This saves you from multiplying big numbers. Also, always check that your answer lies between the two isotopic masses and is closer to the more abundant isotope. That alone eliminates C and D right away.