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MCAT Question A Day - 10/17/14 - Answer!

A natural sample of carbon contains 99% of 12C. How many moles of 12C are likely to be found in a 48.5 gram sample of carbon obtained from nature?

A.  1
B.  4
C.  12
D.  49.5

B is correct. Don't do any complicated calculations. This is the type of problem that everyone will get right, but many will spend too much time trying to be exact. First assume that 100% of the sample is 12C. Now use the formula: moles = grams/molecular weight. This is very close to 4. The 1% that is not 12C is insignificant.

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