# 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

Jun 1, 2018

Theoretically about 0.7 g.

#### Explanation:

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

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

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.