Balance the equation. Note the coefficents on Fe and O2, that is the mole ratio.
calcuate how many moles of Fe, and O2 you have.
Which is the limiting reactant.
For instance, if the coefficents were 45Fe+ 34O2, the mole ratio of O2 to Fe is 34/45
If you had 2.3 moles of Fe, and 3.2 moles of O2, your mole ratio of O2 to Fe would be 3.2/2.3, significantly greater than the coefficents of needed moles. Well, if you have more, that means you have too much O2, so you limiting reageant would be Fe.
Finally, figure out how much product you get with your limiting reageant.
This is a limiting reagent problem. How do I know? Because BOTH reactants are given.
You apparently have the reaction in the problem but didn't post it. I assume it is as follows: Check it to see!
These actually are stoichiometry problems but being limiting reagent we must work them twice and take the smaller product.
4Fe + 3O2 ==> 2Fe2O3
Convert 100 g Fe to moles. moles = grams/molar mass.
Convert 100 g oxygen to moles. Same method.
Using the coefficients in the balanced equation, convert moles Fe to moles of the product.
Same process convert moles O2 to moles of the procuct.
It is most likely that the two answers for moles product formed will not be the same which means one of them is wrong; the correct answer in limiting reagent problems is ALWAYS the smaller value and the reagent producing that number is the limiting reagent.
Now convert the value from the last step to grams. g = moles x molar mass.