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