KJ per hydrocarbon "empirical formula unit"

[Ritzycat]

Homework Statement

A sample of a hydrocarbon is combusted completely in O2(g) to produce 19.65gCO2(g), 4.023gH2O(g), and 280kJ of heat.

Calculate the value of ΔH∘f per empirical-formula unit of the hydrocarbon.

Homework Equations

None I know of

The Attempt at a Solution

Previous calculations found that the hydrocarbon had a mass of 5.811g.

Per empirical formula unit: (I determined the empirical formula to be CH, which was correct)

molar mass of CH = 13.018g/mol

5.811g / 13.018g/mol = 0.447 mol CH

280kJ/0.447 mol = 627.266 kJ/mol

My final answer was 627.266 kJ/mol but that was not correct.

Am I interpreting the meaning of "empirical-formula unit" correctly?

Any help is greatly appreciated.

 

[Borek]

Looks OK to me, perhaps it is just a matter of significant figures, or some rounding error?