# Air is made up of only 2 gases oxygen and nitrogen

1. Oct 23, 2009

### Dell

if air is made up of only 2gases oxygen and nitrogen, and the molecular mass of air is 29g/mol, find the % of each component in the air,

what i thought i should do is make 2 equations,

Mw(O)=16
Mw(N)=14

X*16 + Y*14 = 29 (in 1mol of air)
X + Y = 1

but the solution to this is not possible, i get one as a negative pct

the correct answer is meant to be
72.5%
27.5%

2. Oct 23, 2009

### PhaseShifter

Re: chemistry

These are atomic weights, not molecular weights.
Remember oxygen and nitrogen are both diatomic gases.

3. Oct 23, 2009

### Dell

Re: chemistry

thanks, so my equations would go

X*2*16 + Y*2*14 = 29 (in 1mol of air)
X + Y = 1

then i get 75%, 25%
is this correct? the "correct" answer in my book is 72.5% 27.5%

4. Oct 23, 2009

### PhaseShifter

Re: chemistry

Is that 72.5% by weight, or by number of molecules?

5. Oct 23, 2009

### Dell

Re: chemistry

it actually doesnt say, never thought of that,

is what i got by mol;ecules? how would i make the conversioin

6. Oct 23, 2009

### Dell

Re: chemistry

got it,,

0.75*28=21g
0.25*32=8g

8/29=27.6%
21/29=72.4%

close enough

7. Oct 23, 2009

### Staff: Mentor

Re: chemistry

25% and 75% is a correct answer using 28, 32 and 29 as molar masses and assuming final result must be just molar fraction.

27.6% and 72.4% is a correct answer in terms of mass percentage.

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