Question

Combustion analysis, Empirical and Molecular formulas, help!?


When 2.686 grams of a hydrocarbon, CxHy, were burned in a combustion analysis apparatus, 9.224 grams of CO2 and 1.511 grams of H2O were produced.

In a separate experiment, the molar mass of the compound was found to be 128.2 g/mol. Determine the empirical formula and the molecular formula of the hydrocarbon.

Enter the elements in the order presented in the qustion.

Answer 1

empirical formula is the smallest whole number ratio

molecular formula is the empirical doubled i believe.

Answer 2

you would divide the grams of each molecule produced by the smallest ratio to find the subscripts.

Answer 3

Start from a balanced equation

CxHy + (x+y/4)O2 -> xCO2 + y/2 H2O

so 1 mole of CxHy (molar mass 12x+y)produces
x mole of CO2 and
y/2 mole of H2O

so
2.686 g/(12x+y) moles

produces
9.224/44 =x moles of CO2
so x=0.2096

and produces

1.511/18=y/2 moles of H2O
so y=0.1679

so the ratio is
x:y
0.2096:0.1679
or 5:4

Thus the empirical formula or the simplest formula is
C5H4
for which molar mass is 64 g

we are told in the question that the molar mass is 128.2 g so the molecular formula (ie. the formula for one mole) must be C10H8, that is twice the empirical formula in this case.