Question

a molecule is found to contain 47.35%by mass C, 10.60% by mass H, and 42.05%by massO. what is the empirical formula for the molecule?

Answer 1

In 100g of the molecule, there are 47.35 g of C, 10.60 g of H and 42.05 g of O.

Convert each of these into number of moles.

So, there are
47.35/12 = 3.946 mol of C
10.60/1 = 10.60 mol of H
42.05/16 = 2.628 mol of O.

Divide all these by the smallest molar value (in this case 2.628) to obtain the following ratio
C : H : O = 1.5 : 4 : 1

[Make sure you keep enough accuracy figures in order not to lose accuracy in the end. When you get the ratios, you can throw away figures after the 2nd decimal place cos there are always rounding errors for questions at this level.]

As the molecules do not exist in parts (they exist as a whole),

So, C : H : O = 1.5 : 4 : 1

= 3 : 8 : 2.
So the empirical formula is C₃H₈O₂.