Chemistry

Question Analysis
This question combines solving for unknown isotope abundances with stoichiometric counting of atoms in a sample.
The main focus is on applying the weighted average formula and using percent abundance to find the number of specific isotope atoms.

Key Concept Explanation
Percent abundance = (number of isotope atoms / total atoms) × 100
Number of isotope atoms = total atoms × (percent abundance / 100)

Step-by-Step Solution
Part 1: Find % abundance of Y-63
1. Let x = % abundance of Y-63 → % abundance of Y-65 = 100 - x
2. Set up the atomic mass equation:
63.546 = (62.930 × x/100) + (64.928 × (100 - x)/100)
3. Multiply by 100:
6354.6 = 62.930x + 6492.8 - 64.928x
4. Combine like terms:
6354.6 = 6492.8 - 1.998x
5. Solve for x:
1.998x = 6492.8 - 6354.6 = 138.2
x = 138.2 / 1.998 ≈ 6...