Chem 142, autumn 2011
3. methanol (methyl alcohol or wood alcohol) is a potential automotive fuel, either in
pure form or mixed with gasoline. some fleet vehicles, such as municipal buses, have
been modified to burn methanol-containing fuels. these fuels are also used by some
race cars. a particular automobile modified to use a mixture of 85.0% methanol and
15.0% gasoline by mass gets 25.5 mi/gal. the fuel has a density of 0.775 g/ml. how
many kilograms of methanol does the automobile consume in a trip of 808 km?

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.