Solving Two Difficult Homework Probs: Help Needed!

[helloeveryone]

Homework Statement

Really appreciate helps from anyone, these two probs hurt my head
1. Complete combustion of a 4.10g sample containing only propane (C3H5) and pentane (C5H12), yielded 12.42g CO2 and 6.35g H2O. What is the mass % propane in this sample?
a.4.50% b.38.0% c. 50.0% d.80.0% e.30.0%

2. Balance the following equation in basic solution:
Cr^(3+) + O2^(2-) = CrO4^(2-)
the sum of the coefficients is:
a.38 b.27 c.53 d.18 e.49

Homework Equations

C3H5 + (17/4)O2 = 3CO2 +(5/2)H2O
C5H12 + 8 O2 = 5CO2 + 6H2O

The Attempt at a Solution

For the first prob, i tried to put x is the mole of C3H5 and y is the mole of C5H12. Then i have sum of their mass equal 4.10g. On other part, i have the moles of CO2 from two equation as the factor of x and y, and then get the sum of them equal 12.42g. I solved the equation systems but get the x is negative? not sure what i did wrong? If anyone know please tell me, otherwise i think the problem author gave the wrong information.
For the second prob, nothin i can do, i don't know if O2 is reduction or oxidation agent, don't even think O2^(2-) even exists, but my instructor seems pretty sure that equation can be balanced. Very thankful for the help from anyone.

 

[Bohrok]

First you need one correct and balanced equation with integer coefficients. That propane needs to be C₃H₈
The left side will have propane and pentane with O₂ and the equation will be much easier to work with.

Your first equation with x and y is good. For the other, convert that mass of CO₂ to moles first so you're working with just moles. Then solve the system of equations and convert the moles x and y to masses to calculate the % by mass. Haven't done it myself but it looks like you're on the right track.

Can't help you with the other question, but O₂^2- exists and it is the peroxide ion.

 

[helloeveryone]

propane is C3H8! Damn! 2 hours hitting my head with this prob for nothin until now, i should have learned in heart those shameful, ridisculous organic names. thanks bohrok.
i figured out balanced equation for #2 is 4(OH)^- + 2Cr^(3+) + 3O2^(2-) = 2CrO4^(2-) + 2H2O. but 4 + 2 + 3 + 2 + 2 = 13 for coefficient, not in multiple choices? i guess the author this prob has some trouble with adding... or maybe just me -_-
If anyone points out the wrongness in my balance, very thankful!

 

[Borek]

What is O₂^2-? Such ion is present in some ionic oxides, but not in solutions.

2Cr³⁺ + 3O₂^2- + 4OH^- -> 2CrO₄^2- + 2H₂O

is correct, and 13 it is.