If 75.0% of the isotopes of an element have a mass of 35.0 amu and 25.0% of the isotopes have a mass of 37.0amu what is the atomic mass of the element

A) 35.0amu

B) 35.5amu

C) 36.0amu

D) 37.0amu

Calculating average atomic mass is exactly like calculating a weighted average. Perform the following calculation:

(mass1) (percentage1) + (mass2) (percentage2) = average atomic mass

(35.0) (0.75) + (37.0) (0.25) = average atomic mass

Make sure your percentages are in decimal form for this calculation.

One of the other answers given is correct, though the explanation is lacking a bit. Two of the answers can be eliminated immediately: 35.0 amu and 37.0 amu cannot be the average. If the mixture of isotopes was 50% and 50%, then 36.0 amu would be correct; however, the mixture is 75/25. This leaves only one possible answer choice.