Finding Average Atomic Mass

Question

Hey there! So i have 3 different atomic masses and percentages.

1.) atomic mass: 75.23amu & 24.1%

2.) atomic mass: 74.61amu & 48.7%

3.) atomic mass: 75.2amu & 27.2%

when i’m multiplying them together, i’m using the sig. fig. rules and counting up to 3 significant figures. that leaves me with 74.9199 and using the sig. fig. rules, it becomes 74.9.

but on my worksheet, it’s stating that the answer is: 74.92.

what am i doing wrong??

Answer 1

I recommend asking your instructor about this one, since I think you're correct with 74.9. After you multiply the atomic masses by their percentages, you must apply the multiplicative rule for sig figs before doing any adding. I end up with:

75.23 * 0.241 = 18.1

74.61 * 0.487 = 36.3

75.2 * 0.272 = 20.5

with all results rounded properly to 3 sig figs.

Then when you add these to get the average atomic mass, the sig fig rule is that the final answer can only have as many decimal places as the least precise term (whatever term has the fewest places after the decimal).

18.1 + 36.3 + 20.5 = 74.9

In this case all three of our starting terms have 1 decimal place, so it's correct for your final answer to have just one as well.