Question

Consider the molecule stearic acid (C18H36O2 and Mm = 284. g/mol) which is an analogue of a carcass fat found in the humps of certain camels. Suppose that complete metabolic combustion of a 0.306g sample of stearic acid raises the temperature of a 186.mL sample of hexane (serving as a standard calorimeter) by +2.79 ºC. Given that the specific heat of liquid hexane is 2.30 J/gºC, and its density is 0.655 g/mL, calculate the molar enthalpy of combustion for stearic acid in kJ/mol.

Hcomb = ____ ____kJ/mol

Answer #1

**we know that**

**mass = density x volume**

**so**

**mass of hexane = 0.655 x 186**

**mass of hexane = 121.83 g**

**now**

**heat absorbed by hexane = mass x specific heat x temp
change**

**so**

**heat absorbed by hexane = 121.83 x 2.3 x
2.79**

**heat absorbed by hexane = 781.78311 J**

**now**

**this heat is given by stearic acid**

**so**

**heat given by stearic acid = 781.78311 J**

**now**

**we know that**

**moles = mass / molar mass**

**so**

**moles of stearic acid = 0.306 / 284**

**moles of stearic acid = 0.00107746**

**now**

**molar enthalpy of combustion of stearic acid = heat /
moles**

**= 781.78311 / 0.00107746**

**= 725576.5 J / mol**

**= 725.5765 kJ / mol**

**so**

**the molar enthalpy of combustion of stearic acid is
725.5765 kJ / mol**

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