Question

A compound containing only C, H, and O, was extracted from the bark of the sassafras tree. The combustion of 48.9 mg produced 133 mg of CO2 and 27.2 mg of H2O. The molar mass of the compound was 162 g/mol. Determine its empirical and molecular formulas.

Answer 1

1)

let in compound number of moles of C, H and O be x, y and z respectively

Number of moles of CO2 = mass of CO2 / molar mass CO2

= 0.133/44

= 3.023*10^-3

Number of moles of H2O = mass of H2O / molar mass H2O

= 0.0272/18

= 1.511*10^-3

Since 1 mol of CO2 has 1 mol of C

Number of moles of C in CO2= 3.023*10^-3

so, x = 3.023*10^-3

Since 1 mol of H2O has 2 mol of H

Number of moles of H = 2*1.511*10^-3 = 3.022*10^-3

Molar mass of O = 16 g/mol

mass O = total mass - mass of C and H

= 0.0489 - 3.023*10^-3*12 - 3.022*10^-3*1

= 9.605*10^-3

number of mol of O = mass of O / molar mass of O

= 9.605*10^-3/16.0

= 6.003*10^-4

so, z = 6.003*10^-4

Divide by smallest to get simplest whole number ratio:

C: 3.023*10^-3/6.003*10^-4 = 5

H: 3.022*10^-3/6.003*10^-4 = 5

O: 6.003*10^-4/6.003*10^-4 = 1

So empirical formula is:C5H5O

Answer: Empirical formula is C5H5O

2)

Molar mass of C5H5O,

MM = 5*MM(C) + 5*MM(H) + 1*MM(O)

= 5*12.01 + 5*1.008 + 1*16.0

= 81.09 g/mol

Now we have:

Molar mass = 162.0 g/mol

Empirical formula mass = 81.09 g/mol

Multiplying factor = molar mass / empirical formula mass

= 162.0/81.09

= 2

So molecular formula is:C10H10O2

Answer: molecular formula is C10H10O2