Homework help!! Please give me answer. I keep getting it wrong

Question

Given the following equation:

Fe₂O₃ + 3 C →→ 2 Fe  + 3 CO

How many grams of Fe₂O₃ are required to react 8.62 grams of Fe? 

Answer 1

Fe₂O₃ + 3 C ==> 2 Fe  + 3 CO

molar mass Fe₂O₃ = 159.7 g / mole

atomic mass of Fe = 55.85 g / mole

Assuming you meant to ask how many grams of Fe₂O₃ are required to produce 8.62 grams of Fe, we proceed as follows:

Use the mole ratios in the balanced equation and dimensional analysis:

8.62 g Fe x 1 mol Fe / 55.85 g x 1 mol Fe₂O₃ / 2 mols Fe x 159.7 g Fe₂O₃ / mol = 12.32 g Fe₂O₃ required

Rounding to 3 sig. figs., we have 12.3 g Fe₂O₃ required

Answer 2

Hello Treina C.,

I have to assume you meant, "How many grams of Fe₂O₃ are required to react to yield 8.62 grams of Fe? ".

So what do you need, to convert masses (grams) back and forth to moles (mol), for any chemical material? The molar mass, right? So what is that for Fe, and for Fe₂O₃ ? For Fe, just look up the atomic mass (that's the larger number on the Periodic Table, it includes everything in the nucleus. The mass of the electrons is negligible: mass of 1 electron =~ 1/1836 mass of 1 proton. So we use the atomic mass value as is for all calculations of mass of an atom.). OK, you did that. So divide the Fe mass stated, 8.62 g, by that value, ~55.845 g/mol, to get how many mol of Fe are required in the equation. Next, look at the coefficients in the (balanced!) equation. note that ONE Fe₂O₃ will produce TWO Fe as products.

Now, think about what the balanced equation stands for. Yes, it's a text string, that doesn't help you! Also, it could stand for taking those NUMBERS of atoms or molecules and producing those NUMBERS of atoms or molecules. But it can also stand for taking those MOLES of atoms or molecules and producing those MOLES of atoms or molecules. The only thing it does NOT do, is take those MASSES of atoms or molecules and produce those MASSES of atoms or molecules, because the proportioning of the mass of each item in the equation to the moles of that item is an individual thing.

So now, you have in hand the number of moles of Fe you end up with. But from the leading coefficients in the equation, you see that you only start with 1/2 that amount of Fe₂O₃ moles. So divide your moles of Fe by 2, and you have the number of moles of Fe₂O₃ you had to start with. But your final asnwer must be as grams! So, add up the atomic mass of all the atoms in Fe₂O₃ : that's 2*Fe + 3*O . You should get a value somewhere near 158 (g/mol, all numbers in chemistry have units!). So multiply that value (g/mol Fe₂O₃ ) by the mol Fe₂O₃ value you calculated just above, and there (almost!) is you answer in grams Fe₂O₃ . Why do I say, "almost"? You started with 3 significant digits for your input data; you used at least that many significant digits for your molar masses (but, to be on the safe side, use 2 more digits if your Periodic Table shows them); so you must round to 3 signifcant digits in your answer. To show more or fewer digits would be scienfically wrong: showing more implies that you know those extra digits are correct -- but you can't know that, from your input data precision!; to show less implies that you think you could do the reaction and get the stated precision result, without weighing your Fe₂O₃ carefully -- but you couldn't!

Hope that sets things clearer for you, -- Cheers, --Mr. d.