molecular formula

Question

32.0g of a compound of carbon and hydrogen contains 2.18 mol of carbon. The molecular formula of the compound is

Answer 1

Isaiah,

this is just a small math problem. You must convert back and forth from mol to weight, using the atomic weights of carbon and hydrogen as needed. Hence, 2.18 mol carbon * 12.0107 g/mol carbon = 26.18 g carbon. Since the compound contained only carbon and hydrogen, the remainder 32.0 - 26.18 = 5.82 g is hydrogen. That's 5.82 g / 1.008 g/mol hydrogen = 5.77 mol hydrogen. NOW (and only now) you can work out the ratio of atoms of carbon : atoms of hydrogen = 2.18 : 5.77 . The result = 1 : 2.65 is close enough to 1 : 2+2/3 = 1 : 8/3 to indicate that the empirical formula of the material in question is 3:8 so that would be C₃H₈ . In this particular instance, you KNOW that this is also the molecular formula, b/c doubling to C₆H₁₆ results in an impossible molecule.

(But IF the empirical formula had been C₃H₆ instead, then doubling to C₆H₁₂ would have been a possible molecule -- and therefore the true molecular formula would NOT have been determinable from the data given!)

Hope this helps you understand that shifting back and forth between weights and moles is a very necessary skill.