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MCAT Question A Day - 2/20/14 - Answer!

If 22.5 g of NiCl2 reacts completely with a sufficient amount of NH3 and NaBr to form Ni(NH3)2Br2Cl 2, which of the following expressions gives the number of moles of product that are formed? (at. wt. of Ni = 58.7, of Cl = 35.5, of Br = 79.9)?

A.  22.5 / 79.9
B.  13 / [79.9 + 2(35.5)]
C.  (2)(22.5 / 79.9)
D.  22.5 / [58.7 + 2(14) + 6 + 2(35.5)]

The correct answer is (B). This expression gives the number of moles of NiCl2 (and therefore of Ni). It must equal the number of moles of product, which has 1 Ni per molecule. There is no need to work with the MW of the product, since its weight is not requested.

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