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MCAT Question A Day - 10/20/14 - Answer!

What is the empirical formula of a neutral compound containing 58.6% oxygen, 39% sulfur, 2.4% hydrogen bmass?

A.  HSO3- 
B.  HSO4-
C.  H2SO3
D.  H2SO4

C is correct. We start by assuming a 100 gram sample. By dividing grams by molecular weight, we obtain moles. 58.6/16 = 3.6, 2.4/ 1 = 2.4, 39/ 32 = 1.2. Now we divide through by the lowest number of moles: 3.6/ 1.2 = 3; 2.4 / 1.2 = 2; 1.2/1.2 = 1. This gives you the molar ratio of eacelement. Just to reduce the necessary calculations, the question tells you that it is a neutral compound. Nevertheless, MCAT questions with thimuch calculation occasionally come up, but they are few and far between. Maybe three on one entire exam.

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