Glycerol is a thick, sweet liquid obtained as a byproduct of the manufacture of soap. Its percent composition is 39.12% C, 8.75% H and 52.12% O. Determine the empirical formula for glycerol.

Answer 1

We are given:

C = 39.12%

H = 8.75%

O = 52.12%  

Assume that there is 100 grams of glycerol so that the percentages and grams will be equivalent.

C = 39.12 g

H = 8.75 g

O = 52.12 g

Now we must convert from grams to moles using molar mass of each element.

C = 39.12 g * (1 mol C / 12.01 g C) = 3.25729 mol C

H = 8.75 g * (1 mol H / 1.00784 g H) = 8.68193 mol H

O = 52.12 g* (1 mol O / 15.999 g O) = 3.25770 mol O

What you need to do next is identify which mole is the smallest (in this problem, C is has the smallest moles) and divide all moles by that smallest value.

3.25729 mol C / 3.25729 mol = 1

8.68193 mol H / 3.25729 mol = 2.67 ← not a whole number

3.25770 mol O / 3.25729 mol = 1

CH_2.67O ← cannot have decimals

multiply entire formula by 1.5 to get a whole number for H

1.5 * (CH_2.67O) = C_1.5H₄O_1.5

multiply by 2 to get a whole number for C and O

2 * (C_1.5H₄O_1.5) = C₃H₈O_{3 empirical formula}