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MCAT Question A Day - 10/22/14 - Answer!

What is the percent by mass of carbon in CO2?

A.  12%
B.  27%
C.  33%
D.  44%

B is correct. C has 12 g / mol and 0 has 16 g / mol. The total weight of CO, is 44 g / mol. Carbon's weight divided by the total weight is 12 44= 0.27. We multiply by 100 to get 27%.