Ease Into Prep with an MCAT^® Question of the Day

Beryllium has a molar mass of 9.01 grams per mole. To convert the number of grams in the sample to the number of moles, divide the number of grams by the number of grams per mole, like this: 25 g x 1 mol9 g≈2.8 mol. (D) is correct.

Expert Approach:
I recognize this as a simple mass-to-moles calculation—frequently-tested on the MCAT. To convert grams of beryllium to moles of beryllium, I need beryllium's molar mass. Since it's not given in the question stem, I look for it in the periodic table, and find that it's about 9 g/mol. Then I divide the number of grams of the sample by this molar mass. Of course, I don't know off the top of my head exactly what 25 divided by 9 is, but rather than trying to do time-consuming long division, I mentally estimate: 25 divided by 10 is 2.5, and dividing 25 by a slightly smaller number should give me an answer slightly greater than 2.5. There's only one value greater than 2.5 moles in the answer choices: (D) is correct.It's interesting to recognize one of the traps here: the value in (A) would result from dividing 9 g/mol by 25 g, rather than the other way around. One way to avoid this kind of mistake is to keep track of units throughout any calculation. For example, in this case, if I'd made the erroneous calculation in (A), the resulting units would have been mol-1 rather than mol, so I could have realized there must have been something wrong with my calculation. Another way to avoid this kind of mistake is to simply to think about the quantities involved in a calculation in everyday terms: If a box of oranges weighs 9 pounds, and I have 25 pounds of oranges, they will fill just over two and one half boxes—this is the same problem as the one in this question!

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