[tntgsh]

I've tried to solve them several times, but I couldn't manage to do it, so, if you guys could help, I'd appreciate it.
These are from a test to win scholarships abroad, so I already have the answers, but I don't know how to solve them...

1) A 40,0L sample of N₂ gas containing SO₂ gas as an impurity was bubbled through a 3% solution of H₂O₂. The SO₂ was converted to H₂SO₄:

SO₂ + H₂O₂ H₂SO₄

A 25,0mL portion of 0,0100mol/L NaOH was added to the solution, and the excess base was back-titrated with 13,6mL of 0,0100mol/L HCl. Calculate the ppm of SO₂ (that is, mL SO₂/10^6mL sample) if the density of SO₂ is 2,85g/L. (Atomic weights: H=1 N=14 O=16 Na=23 S=32 and Cl=35,5).

2)Exactly 4,32g of oxygen gas was required to completely burn a 2,16g sample of a mixture of methanol and ethanol. Answer the following questions (a) and (b). (Atomic weights: H=1 C=12 and O=16)
a)How many moles of ethanol are contained in the sample?
b)What is the percentage by weight of methanol in the sample? Write the percentage to two significant figures.


By the way, the answers are:
1) 32,0 ppm
2) a)0,04 mol  b)15%

Thanks a lot 

[Borek]

Please read forum rules.

Show what you have tried.

[tntgsh]

Well, I could do number 1.

About number 2, this is what I got so far...
The equations:

methanol  2CH₄O + 3O₂  2CO₂ + 4H₂O
              2       :     3
ethanol  C₂H₆O + 3O₂  2CO₂ + 3H₂O
              1     :    3

4.32g O₂...n=4.32/32 = 0,135 mol O₂

methanol = 32u
ethanol = 46u

m_methanol + m_ethanol = 2.16

m=n*M,  m_met=32*n_met   and   m_eth=46*n_eth

so,

32*n_met + 46*n_eth = 2.16

And now I stuck...

[Borek]

How many moles of oxygen required to burn n_met?

How many moles of oxygen required to burn n_eth?

This gives you second equation.

[tntgsh]

Ah, thanks a lot, I dit it!