posted by Chris .
If you divide the molar mass of a compound by the empirical formula mass, what is the result?
That division should be VERY close to a whole number. Usually, 0.9 to 1.1 or 1.9 to 2.1 (all depending upon the accuracy and precision of experimental data). That number is the number used to indicate the units of the empirical formula that are hooked together. For example, the molar mass of acetylene is 26. The empirical formula is CH and the empirical formula mass is 13. So, 26/13 = 2.0 which tells me the molecular formula is two units of the empirical formula. Since the empirical formula is CH, then the molecular formula must be (CH)2 or C2H2. If your "number" isn't EXACTLY a whole number, round it so it is. If it is outside the limits I show above, there must be an error in experimental data or the calculation is wrong. I hope this helps.