<p>Chemistry Problem on Mass Percentage?</p>
<p>Can someone answer the following question:</p>
<p>You heat 3.970g of a random mixture of Fe3O4 and FeO in excess O2 to form 4.195g of Fe2O3. The mass percent of FeO originally in the mixture was (Hint: Don't need to balance a chemical equation):</p>
<p>a) 12.1%
b)28.7%
c) 71.3%
d)87.9%</p>
<p>answer is b</p>
<p>Can you show how you did it?</p>
<p>Ooo, that's gonna be long. I have a test at 8:00pm. I'll get back to you when I'm done with it.</p>
<p>Alright thanks...I just want to understand how to do this problem.</p>
<p>Alright, I'm back, now if you can wait for my response, do so, as it'll be a bit long</p>
<p>Your first objective is to find how much Fe is in the product, so you find the % composition of Fe in Fe2O3, so it'll be
(55.845x2)/[(55.845x2)+(16x3)]x100%= 69.94% Fe
Then you multiply it with the mass of the Fe2O3 to get
.6994x4.195g=2.934g Fe</p>
<p>Now comes the tricky part, and will take me a while to explain.
You have 3.970g of a mixture of FeO and Fe3O4, with x% FeO and (100-x)% Fe3O4 that somehow combines with O2 to get Fe2O3</p>
<p>Since Oxygen is in excess we will ignore it and focus on the Fe for the two compounds
We will let x be the mass of the Fe3O4 and y be the mass of the FeO, so we have
x+y=3.970
Our ultimate goal here is to find y, so we express x in terms of y
x=3.970-y</p>
<p>Now, we look at the two compounds and say "They are made up of different elements, so the percent compositions of Fe must be different." So now we find the percent composition of Fe for the two compounds.
% for Fe3O4 = (55.845x3)/[(55.845x3)+(16x4)]x100%=72.36%
% for FeO = (55.845)/[(55.845)+(16)]x100%=77.73%</p>
<p>Now what does this all mean? It means we can set up another equation since we know that the total Fe between the two compounds is 2.934g
We can now say
0.7236x+0.7773y=2.934g
This is because x is the mass of Fe3O4, and since we know the % composition of Fe, we know that 72.36% of x = the amount of Fe in the Fe3O4. The same reasoning applies with the FeO.</p>
<p>I'll post this up so you can read up on it earlier.</p>
<p>Now we have two equations and two unknowns, so what do we do? Solve for y!
We know that x=3.970-y so get rid of any x's in the second equation
0.7236(3.970-y)+0.7773y=2.934
Now we have
2.873-0.7236y+0.7773y=2.934
And so
0.0537y=0.061
And so
y=1.136g</p>
<p>Now we just divide it by the mass of the mixture and multiply by 100% to get the percent mass and we're done.
1.136/3.970 x100%=28.6%
Slightly off due to roundoff error, but there it is.</p>
<p>Thanks so much, I really appreciate it. Good explanation.</p>
<p>There’s not any other way to do this problem?</p>