Empirical and molecular formulas of cumene

Question

Cumene containing only carbon and hydrogen is used in the production of acetone and phenol, combustion of 47.6mg cumene produces some CO2 and 42.8mg water. The molar mass of cumene is between 115 and 125 g/mol. determine the empirical and molecular formulas.

please show work

Answer 1

let's find the mass of both elements:

42.8 mg H2O @ 2 mg Hydrogen / 18 mg H2O = 4.76 mg Hydrogen

47.6 mg total - 4.76 mg H = 42.8 mg carbon

let's find the moles of both:

4.76 mg @ 1 mg/ mmol = 4.76 mmol Hydrogen

42.8 mg C @ 12 mg / mmol C = 3.57mmol carbon

let's get the ratio of moles:

4.76 / 3.57 = 1.33  mole Hydrogen

3.57 / 3.57 = 1 mole Carbon

we triple them to get whole numbers:

your first answer: empirical formula is C3H4

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C3H4 has a formula mass of 40

what with the molar mass of cumene is between 115 and 125 g/mol.

(an average of 120g/mol) / 40 g / formula = 3 fold larger

last answer : Molecular formula is C9 H12