Question

Caffeine consist of carbon, hydrogen oxygen and Nitrogen. When 0, 1920g of caffeine combost in axcess of Oz, forms 0,3482g CO2 and 90891q H2O Caffeine consist of 2884 % mars N and the molermass is between 190 and 2009/mol What is caffeine molekular formel? Answere CG H o NyO2.
can someone explain all steps on how to solve this thx

Answer 1

We must do an analysis:
CO2 and H2O are obtained from a combustion reaction, from caffeine and oxygen in excess, therefore, to form the products the limiting reagent was the amount of carbon and hydrogen present in the caffeine, we can calculate the moles of carbon and of hydrogen involved to know the existing quantity in the compound.

Then, knowing the % of nitrogen in the sample we can calculate the mass of nitrogen, knowing these 3 species (carbon, hydrogen and nitrogen) we can calculate the amount of oxygen.
Having the mass quantity of all the elements, we can calculate the% of each to find the empirical formula of the compound, and then the molecular one.

We start with:

Then:

If:

For oxygen:

Taking the masses of all the components, we calculate the% of each in the caffeine:

We divide the % by the atomic weights:

We take the smallest value, in this case that of oxygen, and divide the others by it to have the ratio in the empirical formula:

So the empirical formula is: C₄H₅N₂O₁ (97.103g/mol)

Knowing that the molecular weight ranges from 190 to 200, multiplying the empirical formula by 2:

C₈H₁₀N₄O₂ (194.206 g/mol).