KJ per hydrocarbon "empirical formula unit"

AI Thread Summary
The discussion focuses on calculating the standard enthalpy of formation (ΔH∘f) per empirical formula unit of a hydrocarbon after complete combustion. The user determined the empirical formula to be CH and calculated the molar mass as 13.018 g/mol. They found the number of moles of CH from the hydrocarbon's mass and then calculated ΔH∘f as 627.266 kJ/mol. However, this result was deemed incorrect, prompting questions about the interpretation of "empirical-formula unit" and potential issues with significant figures or rounding errors. Clarification on these points is sought to resolve the discrepancy in the calculation.
Ritzycat
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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.
 
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Looks OK to me, perhaps it is just a matter of significant figures, or some rounding error?
 
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