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

The answer 74.92 would be correct if you rounded your final answer to 4 sig figs and 74.9 would be correct if you rounded your final answer to 3 sig figs. All of the masses for the isotopes in the problem were measured, so the answer with 3 sig figs would be correct unless the problem clearly stated that it wanted you to round the final answer to 4 sig figs. The rules of sig figs when adding or subtracting clearly state that the amount with the least # of sig figs or the amount with the least # of sig figs after the decimal determine how many sig figs the final answer should have. So, your answer is right, according to the rules that govern determining sig figs. Since, 75.2 amu only goes to the tenth place, your answer must be rounded to the nearest tenth according to the rules.