Thallium consists of 29.5% TI-203 and 70.5% TI-205. What is the relative atomic mass of thallium?
This is basically a ratio math problem. We have two different masses and a known percentage of each. Combining them into a weighted-average mass is done by multiplying each mass by the weight percent and adding them up.
0.295 x 203 + 0.705 x 205 = 59.885 + 144.525 = 204.4
This is what we would expect – a slight “shift” to the higher mass from the 50/50 average of 204 because of the larger percentage of Tl 205 in the sample.