[sega18]

Hello everyone,

I'm a bit confused about some of these more difficult Stoichiometry problems I've been doing. Here is one:

4FeO + O₂ -> 2 Fe₂O₃

4Fe₃O₄ + O₂ -> 2 Fe₂O₃

Mass of the mixture of 4Fe₃O₄ and FeO = 5.430 g and mass of products = 5.779 g.

The first step (after writing and balancing the equations) is to define a variable or 2. My grandpa suggested that x = "moles of reaction 1" and y = "moles of reaction 2". The internet suggested that x = fraction of mixture that is FeO. My book suggests that x = mass FeO (or mol FeO).

What the hell is the best way to solve this problem? I'm totally confused.



Also, there's another problem that I understand how to do, but do not understand why it works:

NaCl + Ag⁺ -> AgCl
KCl + Ag⁺ -> AgCl

Mixture = 4.000 g and product = 8.5904 g.

I can do x + y = 4 and x/58.44 + y/74.55 = mol Cl

Why can't I do something so relatively simple for the above problem?

Thanks for all your help,

Segev 

[UG]

My book suggests that x = mass FeO (or mol FeO).

I like the look of this Personally, I would let x = mole of FeO. These kind of problems can be done very systematically, are there no examples in your textbook?
For example, your second problem, this is how I would do it:
m(NaCl) + m(KCl) = 4.000 g
n(NaCl) + n(KCl) = n(AgCl)
You can easily calculate n(AgCl). Now we just need to manipulate the first equation slightly using the relationship between moles (n), mass (m) and molar mass (M). We can substitute mass with moles x molar mass, so we end up with:
58.45 x n(NaCl) + 74.55 x n(KCl) = 4.000 g
You now have two equations with two variables, so you can solve to find moles of NaCl and KCl. The first problem can be done the same way.