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MCAT Question A Day - 10/31/14 - Answer!

If the density of a gas is given as p which of the following expressions represents the molecular weight of the gas?

A.  Pp/ RT
B.  pRT / P
C.  nRT / Pp
D.  Pp / RT

B is correct. Since density (p) is mass (m) divided by volume (V), and mass is moles (n) times molecular weight (MW), we have (nMW)/V ~ p. After some algebra we have: MW ~ (pV)/n. From the ideal gas law we know that V/n ~ RT/p. Substituting we have answer B.

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