Question

You are searching for an inexpensive synthetic route to make anthraquinone to use it as the active component in a new laxative formulation. Reaction of benzene and phthalic anhydride yielded a yellow solid thought to be anthraquinone. Combustion analysis of 0.520 g of the yellow solid yielded 1.54 g of carbon dioxide and 0.180 g of water; (all carbon dioxide and water formed by combustion were isolated and weighed). In another experiment, the molar mass of the yellow solid was found to be approximately 205 g/mol. Use the analysis information above to determine the molecular formula of the yellow solid.

Answer 1

moles of CO2 = 1.54 / 44 = 0.035

moles of C = 0.035

mass of Carbon = 12 x 0.035 = 0.42 g

moles of H2O = 0.180 / 18 = 0.01

moles of H = 2 x 0.01 = 0.02

mass of hydroegen = 0.02 g

Oxygen mass = 0.520 - (0.42 + 0.02 ) = 0.08 g

moles of O = 0.08 /16 = 0.005

C                H              O

0.035      0.02        0.005

    7                4              1

C7H4O -----------------------> empirical formula

mass of empirical formula = 104 g

molar mass of yellow solid = 205 g

n = molar mass / emperical formula mass = 205 / 104

    = 2

molecular formula = 2 x C7H4O

molecular formula = C14H8O2