[noiseordinance]

Ok, I got another mole question, but phrased in a different way and I'm drawing blanks here...

Menthol (M = 156.3 g/mol), a strong-smelling substance used in cough drops, is a compound of carbon, hydrogen, and oxygen. When 0.1595g of menthol was subjected to combustion analysis, it produced 0.449g of CO₂ and 0.184g H₂O. What is menthol's molecular formula?

So, I trying to add the CO₂ and H₂O and it doesn't add up to 0.1595g, so I have a feeling I'm missing something from the equation. Now, if I recall correctly, combustion requires oxygen, so would oxygen be the factor that isn't mentioned in this problem?

[nj_bartel]

Yes.  Calculate the moles of water and co2 produced first.

[Arkcon]

You will have to write a balanced chemical equation for the combustion reaction first.  You've got it right, the oxygen in the air is a reactant.  But remember, menthol provides some of the oxygen for the products, you won't know how to account for all reactants and products with pot balancing.

[noiseordinance]

Hehe, I'm not sure I'm grasping it yet... (on a side note, why do instructors only do the easy problems during lectures? ) Anyways... I'm not sure how to write a balanced chemical equation since I don't know the ratio on either side of the equation. I'm guessing it looks something like this:

_CH₂O₃ + _O₂ -----> _CO₂ + _H₂O

Now, I'm guessing that menthol is CH₂O₃ in the first place, but really I have no clue... Argh! LOL

[Arkcon]

Oops, I didn't think that through.  You can't write a proper chemical equation without a formula.  Heh.  It's late, over here.  Anyway, you've done a good job guessing a formula.  Now, you've got grams of carbon dioxide and water.  How many moles of each is that?  The carbon and hydrogen come from menthol, so can you figure out what multiples of carbon and hydrogen you need?  The balance is oxygen, but that you'll figure out later.

[noiseordinance]

Ok, so I figure out the moles of carbon dioxide and water...

0.449g CO₂ / 44.0g = 0.0102 moles
0.184g H₂O / 18.0g = 0.0102 moles

I still don't think I know where to go from here. I'm guessing empirically speaking, there will be equal parts CO₂ and H₂O as the products since they both seem to output the same amount of moles...

Thanks for the help, btw.

[Borek]

What is mass of carbon in produced CO₂?

What is mass of hydrogen in produced H₂O?

By how much original sample was heavier?

Assume this diference to be oxygen - you were told there are no other elements in the compound.

[noiseordinance]

Ok, so can I just take 0.449g of carbon dioxide and 0.184g of water and add them together (0.633g) and then subtract the original 0.1595g of menthol to get the amount of oxygen required, in this case, 0.4735g? Am I thinking this correctly? Argh...

[Borek]

Ok, so can I just take 0.449g of carbon dioxide and 0.184g of water and add them together (0.633g) and then subtract the original 0.1595g of menthol to get the amount of oxygen required, in this case, 0.4735g? Am I thinking this correctly? Argh...

No. Answer EXACTLY questions I have asked.

Not mass of carbon dioxide, but mass of carbon in carbon dioxide.