past paper combustion analysis Q

This is from a June 02 Nuffield paper and I can't figure it out.

When a sample of aspartame mass 0.450g is burnt in oxygen, 0.943g CO2 and 0.248g H20 produced. Show these are consistent with the molecular formula. Molecular formula = C14 H18 N2 05

Now I assume it is a combustion analysis question but my problem is that I can't figure out how to get the nitrogen content.

Reply 1

silent ninja

hmmm... if i'm understanding the question, its not asking for the nitrogen content, just that the masses obtained are consistent with the M_r given. All of the hydrogen is combusted to water, and carbon to CO₂. So without balancing you can confirm whether these values are correct.

The M_r= 294

So hydrogen content in 0.450g= 18/294*0.450= 0.0276g
hydrogen after combustion= 2/18*0.248= 0.0276g

These are the same so its consistent for H₂O produced.
You can do the same for carbon.

Reply 2

thestudent

right, I was just trying to get an empirical formula I think. but that makes more sense. thanks

Reply 3

qwerty89

but why can't you / why arnt you allowed do it for nitrogen and oxygen

Reply 4

silent ninja

From the info given without formula you cant work out the oxygen or nitrogen content. The molecule itself could contain oxygen as well as the oxygen from the air to combust. The question just says 'show' these masses are consistent with the given formula.