Question

what is the relative atomic mass of a hypothetical element that consists of the following isotopes in the indicated natural abundances?
isotope | isotopic mass (amu) | relative abundance (%)
1 | 85.9 | 15.0
2 | 87.9 | 13.0
3 | 91.0 | 72.0
express your answer to three significant figures and include the appropriate units.
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Explanation:

Step1: Calculate contribution of Isotope 1

Multiply isotopic mass by relative abundance (as a decimal).
$85.9 \times \frac{15.0}{100} = 85.9 \times 0.150 = 12.885$

Step2: Calculate contribution of Isotope 2

Multiply isotopic mass by relative abundance (as a decimal).
$87.9 \times \frac{13.0}{100} = 87.9 \times 0.130 = 11.427$

Step3: Calculate contribution of Isotope 3

Multiply isotopic mass by relative abundance (as a decimal).
$91.0 \times \frac{72.0}{100} = 91.0 \times 0.720 = 65.52$

Step4: Sum the contributions

Add the three contributions together.
$12.885 + 11.427 + 65.52 = 89.832$

Step5: Round to three significant figures

The result $89.832$ rounded to three significant figures is $89.8$.

Answer:

Value: $89.8$, Units: amu