Question

Stearic acid (C18H36O2) is a fatty acid, a molecule with a long hydrocarbon chain and an organic acid group (COOH) at the end. It is used to make cosmetics, ointments, soaps, and candles and is found in animal tissue as part of many saturated fats. In fact, when you eat meat, you are ingesting some fats containing stearic acid. ( of C18H36O2 = –948 kJ/mol, CO2=-393.5kJ/mol, H2O=-241.826kJ/mol).
Calculate the heat (q) released in kcal when 2.831 g of stearic acid is burned completely.

Answer 1

The molar mass of stearic acid is 18*AC+36*AH+2*AO=18*12+36*1+16*2=284g/mol.

n=m/M=2.831/284=0.01 moles

C18H36O2+27O2-->18CO2+18H2O

we have 18*0.01=0.18 moles of CO2

18*0.01=0.18 moles of H2O

0.01*948=9.48kJ from stearic acid

0.18*393.5=70.83kJ from CO2

0.18*241.826=43.52kJ from H2O

9.48+70.83+43.52=123.83kJ

123.83*4.184=518.10kcal