# Chapter 2 - Atoms, Molecules, and Ions - Section Problems - Page 74: 110

The mass of the third magnesium isotope is equal to 26 $u$.

#### Work Step by Step

Atomic weight for magnesium : 24.305 $u$ (Periodic table) $(Mass^{24}Mg)(Abundance^{24}Mg)$ + $(Mass^{25}Mg)(Abundance^{25}Mg) + (Mass^{x}Mg)(Abundance^{x}Mg) = Atomic$ $weight$ $(23.985)(0.7899) + (24.986)(0.1000) + (Mass)(0.1101) = 24.305$ $(Mass)(0.1101) = 24.305 - (23.985)(0.7899) - (24.986)(0.1000)$ $Mass = \frac{24.305 - (23.985)(0.7899) - (24.986)(0.1000) }{0.1101}$ $Mass = 25.982\ u$ $Mass \approx 26\ u$

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