Question

A compound which contains hydrogen, oxygen, carbon only has molar mass of about 85 g/mol. When 0.43 g of the compound is burned in excess of oxyfen, 1.10 g of CO2 and 0.45 g of H2O are formed. Find the molecular formula and the empirical formula of the compound

Answer 1

g C = g CO2 x (atomic mass C/molar mass CO2) = 1.10 x 12/44 = about 0.3

g H = g H2O x (2*atomic mass H/molar mass H2O) = about 0.05

g O = 0.43- g H - g C = about 0.08

Convert to mols.
mols C = 0.3/12 = ?
mols H = 0.05/1 = ?
mols O = 0.08/16 = ?

Now find the ratio of these numbers to each other with the smallest being 1.00. The easy way to do that is to divide the smaller numberr by itself, then divide the other numbers by the same small number. Round to whole numbers. I get C5H10O but check all of these numbers to make sure I didn't hit the wrong key of my calculator. That's the empirical formula.
To find the molecular formula, you know the approx molar mass is 85. What's the empirical mass for C5H10O. That will be 5*12 + 10*1 + 16 = 86 so the molecular formula must be 1 unit of the empirical formula; i.e., molecular formula is C5H10O.