Chemistry Question

Question

An element has three stable isotopes. Isotope 1 has a mass of 145.8975 amu and an abundance of 0.18750. Isotope 2 has a mass of 146.8756 amu and an abundance of 0.33750. Isotope 3 has a mass of 147.8565 amu. What is the atomic weight of this element? Report correct number of sig figs.

Answer 1

The atomic weight of the element will be a weighted average of all of the isotopes. The abundance of isotope 3 can be obtained by subtracting the abundance of the other 2 isotopes from 1.00000. Thus, abundance of isotope 3 = 1.00000 - 0.18750 - 0.33750 = 0.47500

(145.8975 amu x 0.18750) + (146.8756 amu x 0.33750) + (147.8565 amu x 0.47500)

27.356 + 49.571 + 70.232 = 147.159 amu