[dangerousdan]

Heres my problem I have a midterm tomorrow and I cannot understand one last problem in reguards to stoich.  in fact I don't even know where to start any help would be greatly appreciated. heres the question..


Q2. [6 points] A commercial method of manufacturing hydrogen involves the reaction of iron and steam according to the reaction shown below.

             3 Fe(s) + 4 H2O(g)   ---->   Fe3O4(s) + 4 H2(g)


        (a) If excess H2O(g) (steam) is used, how many grams of Fe are required to produce 2.06g of H2(g)
   




        (b) How many grams of Fe are consumed in the conversion of 128 g of H2O to H2(g)?







  (c) In one reaction, 3.55 mol Fe3O4(s) were produced, how many grams of H2(g) were

[kataklysmic]

the fact that water (or steam, i guess) is in excess just tells you that the problem is not a limiting reactant question, which makes your life a whole lot easier. i'm not sure if that helps, but judging by the topic title i assumed it was the "in excess" thing that was messing you up.

[dangerousdan]

Well I understand the steam is in excess but where do i start in reguards to the calculations? do I change the grams of H2 to moles and then multiply by the coeficients of Fe and H2 or what am I missing?

[kataklysmic]

yeah, i would try something like this:

   2.06g H2           1mol H2                 3 mol Fe                55.85g Fe
          1                    2.02gH2               4mol H2                  1mol Fe


... i'm really sorry if that's wrong, i'm only in high school and i haven't done stoich for nearly a year now.

[xiankai]

Well I understand the steam is in excess but where do i start in reguards to the calculations? do I change the grams of H2 to moles and then multiply by the coeficients of Fe and H2 or what am I missing?

that's right, once you find moles of H₂ u can convert it to moles of Fe required, (4:3 ratio if you can see), then convert the moles of Fe back to grams.

for (b), its pretty much the same, only that u're dealing with H₂O instead of H2 (4:3 ratio of H₂O:Fe again)

(c) is easier. find moles of H₂ directly and convert...

[charco]

As the reaction is reversible, in fact, an equilibrium it makes sense to have excess H2O to push the equilibrium to the side of the products. Without this an equilibrium mixture would be established preventing the stoichiometry being used for the yield.

[dangerousdan]

thanks guys that helps out a ton!