18.78° C
We are given;
We are required to determine the initial temperature of the calorimeter;
First, we determine moles of ethanol
Moles = Mass ÷ Molar mass
Moles = 12.8 g ÷ 46.07 g/mol
= 0.278 moles
But. molar enthalpy of ethanol is 1360 kJ/mol
Thus,
Heat released, Q = n × ΔHc
= 0.278 mole × 1360 kJ/mol
=378.08 kJ
Assuming the initial temperature of the calorimeter is X
Heat absorbed, Q = C × ΔT, Where C is the heat capacity
Change in temperature, ΔT = (85.7 - X)°C
Therefore;
Q = 5.65 kJ/°C × (85.7 - X)°C
= 484.205 - 5.65 X kilo Joules
Thus;
484.205 - 5.65 X Joules = 378.08 kJ
5.65 X = 106.125
X = 18.78° C
Thus, the initial temperature of the calorimeter 18.78° C