Use this information to calculate the molecule formula for the compound

Question

Nicotine is composed of carbon, hydrogen and nitrogen and its molar mass is about 160g/mol. When 4.38mg of the compound burns completely in oxygen, the reaction produces 11.9mg of gaseous CO₂ (molar mass =44.01g/mol) and 3.41mg gaseous H₂O (molar mass =18.02g/mol) Use this information to calculate the molecular formula for nicotine. 

 

I've converted the mass to moles for the products, giving 2.70 x 10^-4 mol CO₂ and 1.89 x 10^-4 mol H₂O.

I'm just not sure where to go from here... Do I use the approximate molar mass of nicotine to convert the mass to moles of nicotine as well? If so, then how would I carry on with this question because I don't know the mole ratios. 

Answer 1

From the moles of CO2 you can get the moles of carbon (C) = 2.7x10-4 moles C. 

 

From moles of H2O you can get the moles of hydrogen (H) = 1.89x10-4 moles H2O x 2 mol H/mol H2O = 3.78x10-4 moles H. 

 

Now you need to find moles of nitrogen (N). Find mg of C and mg H and subtract the total from 4.38 mg to get mg of N. Convert that to moles N. 

 

Divide all by the smallest number of moles to get whole number moles. If necessary multiply by a constant to get whole numbers of moles. That will be the empirical formula. Find the molar mass of the empirical formula and divide that into 160 to find the number of empirical formulae in the molecular formula. 

 

Multiply the empirical formula by that number to get the molecular formula.