# A fictitious element that has two isotopes has an average atomic mass of 117.3. If 65.43% of the atoms of the element have an atomic mass of 119.8, what is the atomic mass of the other isotope?

##### 1 Answer
Sep 6, 2017

Approx...$113 \cdot \text{amu}$

#### Explanation:

The weighted average of the isotopes is equal to $117.3 \cdot \text{amu}$.

And thus.........with units of $\text{amu}$

[65.43%xx119.8+34.57%xxchi] =117.3

Where $\chi \equiv \text{mass of the other isotope}$....

And so we solve for $\chi$....

$\chi = \frac{117.3 - 0.6543 \times 119.8}{0.3457} = 112.6 \cdot \text{amu}$