Combustion of a #20*g# mass of compound gives #44*g# #CO_2#, and #7.99*g# of water. Given that the molecular mass is #180*g*mol^-1# what are the empirical and molecular formulae?
This question operates under an unspoken assumption. We get (eventually!) a molecular formula of
We can burn organic compounds in a furnace to produce (i) carbon dioxide; and (ii) water. The carbon and hydrogen content of these products COME directly from the hydrocarbon. Sometimes these are expressed as a percentage by mass and SOMETIMES, as here, the measured percentages do not add up to 100%. The missing percentage is presumed to be oxygen, which is NOT measured, but obtained by difference, i.e.
Here, the carbon content is measured by
So let's see if we can get something useful.
We divide thru by the SMALLEST molar quantity, that of oxygen to get a trial formula of
But we know that the
I am sorry for making such a meal of this answer, but if you are an undergrad, you should be aware of the background to this question. If you are at A level, your teacher has no business asking you these sorts of questions.