If the heat of combustion of ethanol is #1367kJmol^-1#, how would you calculate the theoretical mass of ethanol required to heat #200mL# of water from #21.0^@# to #45.0^@#?

1 Answer
Simon Moore
Jun 1, 2018

Theoretically about 0.7 g.

Explanation:

First, work out how much energy is involved in heating the water.

#E=m.c.theta#

E is the energy in J, m is the mass in kg, c is the specific heat of water which is 4186 J/kg.°C and #theta# is the temperature increase. It is fine to assume density of water as 1 #g.cm^-3#

E = 0.2 x 4186 x 24 = 20,092.8 J (or 20.0928 kJ)

1 mole of ethanol when combusted releases 1367 kJ of energy. So to obtain 20.0928 kJ you need (20.0928/1367) = 0.0147 moles.

Molar mass of ethanol is 46.07 g/mol, so you need 0.0147 x 46.07 = 0.68 g of ethanol.

However, this is a theoretical quantity only, as in practise heat energy will be lost to the environment and apparatus during the combustion.

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