The reaction as shown is a combustion reaction. 29.8 kCal of heat is produced when 4.20 g of ethanol reacts with O2 gas.
The equation of this reaction is shown below;
C2H5OH + 3O2 →2CO2(g)+3H2O(l)+327 kcal
From the reaction equation, we can see that 1 mole of ethanol produces 327 kcal of heat.
Now, we have to find the number of moles in 4.20 g of ethanol as follows;
Number of moles = mass/molar mass = 4.20 g/46 g/mol = 0.091 moles
If 1 mole of ethanol yields 327 kcal of heat
0.091 moles of ethanol yields 0.091 moles × 327 kcal / 1 mole
= 29.8 kCal
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