Question

An unknown is discovered to have three isotopes. 25% have a mass of 86.0, 66% have a mass of 89.0, and the rest have a mass of 92.0. What is the average atomic mass?

a) 88.0 amu
b) 87.0 amu
c) 90.0 amu
d) 89.5 amu

Answer 1

The average atomic mass of an unknown element with three isotopes, considering their respective abundances and atomic masses, is calculated to be 88.52 amu. Given the choices, the closest answer is 88.0 amu (option b).

Step-by-step explanation:

To calculate the average atomic mass of an unknown element with three isotopes, we multiply the mass of each isotope by its percent abundance (expressed as a decimal), and then sum up these products. Thus, for the unknown element's isotopes, the calculation looks like this:

• (0.25 × 86.0 amu) = 21.5 amu
• (0.66 × 89.0 amu) = 58.74 amu
• (0.09 × 92.0 amu) = 8.28 amu

Adding these together, the average atomic mass is:

21.5 amu + 58.74 amu + 8.28 amu = 88.52 amu

Considering the given options, the closest answer to our calculation is (b) 88.0 amu, even though the exact calculation gives us 88.52 amu. Sometimes rounding differences can occur, so it's important to choose the closest answer provided in a multiple-choice question.