Asample of an unknown gas effuses in 12.1 min. an equal volume of h2 in the same apparatus under the same conditions effuses in 2.42 min. what is the molar mass of the unknown gas?

MW H2 = 2

MW 2 = ?

rate 1 = 1/12.1

rate 2 = 2.42

aopply graham law

Rate1/rate2 = sqrt(MW2/MW1)

(12.1/2.42)^2 = MW2/2

MW = 2(12.1/2.42)^2 = 50 g/mol

Therefore, the molar mass of the unknown gas is 50 g/mol

n2(g) + 3h2-> 2nh3(g)   this is the balanced equation

note the mole ratio between n2, h2 and nh3.   it is 1 : 3 : 2   this will be important.

moles n2 present = 28.0 g n2 x 1 mole n2/28 g = 1 mole n2 present

moles h2 present = 25.0 g h2 x 1 mole h2/2 g = 12.5 moles h2 present

based on mole ratio, n2 is limiting in this situation because there is more than enough h2 but not enough n2.

moles nh3 that can be produced = 1 mole n2 x 2 moles nh3/mole n2 = 2 moles nh3 can be produced

grams of nh3 that can be produced = 2 moles nh3 x 17 g/mole = 34 grams of nh3 can be produced

note:   the key to this problem is recognizing that n2 is limiting, and therefore limits how much nh3 can be produced.

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