Question

A compound has a percent composition of 40.0% carbon , 6.72% hydrogen , and 53.28% oxygen . If it's molar mass is 180 g/mol , what is its molecular formula ?

Answer 1

1) Molar composition

Base: 100 g

C: 40.00 g / 12.0 g /mol = 3.333 mol

H: 6.72 g / 1 g/mol = 6.72 mol

O: 53.28 g / 16g/mol = 3.33 mol

2) Divide by the smaller number of moles

C: 3.333 / 3.33 = 1.00

H: 6.72 / 3.33 = 2.02 ≈ 2

O: 3.33/3.33 = 1.00

3) Empirical formula

CH2O

molar mass of the empirical formula = 12 g/mol + 2*1g/mol + 16 g/mol = 30 g/,ol

4) Number of times that the mass of empirical formula is contained in the molar mass = 180 g/mol / 30 g/mol = 6

5) Molecular formula = 6 times the empirical formula

=>C6H12O6

Answer: C6H12O6