The combustion of 90.0 g of oxalic acid, C₂H₂O₄(s) in a bomb calorimeter whose heat capacity is 4.60 kJ/°C causes the temperature to rise from 25.0°C to 79.6°C. What is the heat of combustion of oxalic acid?

Question

Quizplus · Accepted Answer

First, we need to calculate the amount of heat absorbed by the calorimeter during the reaction:
q = CΔT
where q is the heat absorbed, C is the heat capacity of the calorimeter, and ΔT is the change in temperature. Plugging in the values, we get:
q = (4.60 kJ/°C) x (79.6°C - 25.0°C) = 247.6 kJ
Next, we need to calculate the amount of heat generated by the combustion of the oxalic acid:
q = -mcΔT
where q is the heat generated, m is the mass of the oxalic acid, c is the specific heat capacity of the oxalic acid (we will assume it is 1.0 J/g°C), and ΔT is the change in temperature. We convert the mass given to grams and the temperature change to Celsius:
q = -90.0 g x 1.0 J/g°C x (79.6°C - 25.0°C) = -4,860 J or -4.86 kJ
Finally, we can calculate the heat of combustion (ΔHcomb) using the equation:
ΔHcomb = -q / n
where n is the number of moles of oxalic acid combusted. To find n, we need to divide the mass of the oxalic acid by its molar mass:
n = 90.0 g / 90.03 g/mol = 0.9998 mol (rounded to four significant figures)
Then we can plug in the values to find ΔHcomb:
ΔHcomb = -(-4.86 kJ) / 0.9998 mol ≈ -251 kJ/mol
Therefore, the best choice is E, with a heat of combustion of approximately -251 kJ/mol.

The Combustion of 90