Question

A sample contains isotopes of Magnesium ${}^{24}\text{Mg}$ and ${}^{25}\text{Mg}$. If the percentage of ${}^{25}\text{Mg}$ in the sample is $20\%$, then what is the atomic mass of Mg?

• A. 24.5
• B. 24.8
• C. 24.2
• D. 24.4

Answer 1

The correct option is C 24.2

The atomic mass of ${}^{25}\text{Mg}$ will be $20\%$ of the sample, so it is $\left(\frac{20}{100}\right)\times 25=5$
The atomic mass of ${}^{24}\text{Mg}$ will be $80\%$ of the sample, so it is $\left(\frac{80}{100}\right)\times 24=19.2$
Therefore, the total mass of magnesium will be $5+19.2=24.2$