A sample of acetone having a mass of 1.920 g undergoes complete combustion in an insulated calorimeter holding 400 mL of water. It is determined that the water and calorimeter together absorb 58.829 kJ of heat energy from this combustion. Based on this information, what is the standard molar enthalpy of combustion for acetone in kJ/mol?
I have tried it and got as far to the mole part. the answer is -1780 kJ/mol but I'm getting like 8.5 kJ/mol when I divide using molar enthalpy change equation, help!
molar mass of acetone ... 58.08 g
moles of acetone ... 1.920 / 58.08
molar enthalpy is energy per mole
To determine the standard molar enthalpy of combustion for acetone, we need to follow a step-by-step approach.
Here's how you can calculate the correct answer:
1. Calculate the moles of acetone used in the combustion reaction.
To do this, we need to convert the mass of acetone (1.920 g) into moles. The molar mass of acetone (C3H6O) is 58.08 g/mol.
moles of acetone = mass of acetone / molar mass of acetone
moles of acetone = 1.920 g / 58.08 g/mol
moles of acetone ≈ 0.033 moles
2. Determine the heat energy absorbed by the water and calorimeter.
The heat absorbed by the water and calorimeter can be calculated using the equation:
q = m × c × ΔT
Where:
q = heat energy absorbed
m = mass of water
c = specific heat capacity of water
ΔT = change in temperature
In this case, we are given that the calorimeter and water together absorb 58.829 kJ (note the units). The mass of water is 400 mL, which is equivalent to 400 grams. The specific heat capacity of water is approximately 4.18 J/(g·°C).
So, q = 58.829 kJ = 58,829 J, m = 400 g, c = 4.18 J/(g·°C)
3. Calculate the change in temperature (ΔT) of the water.
In this problem, we are not given the change in temperature directly. However, we can assume that the heat absorbed by the water is equal to the heat released by the combustion of acetone.
4. Calculate the enthalpy change (ΔH) for the combustion of acetone.
Using the equation q = ΔH × n, we can now calculate ΔH (enthalpy change).
ΔH = q / n
ΔH = 58,829 J / 0.033 moles
ΔH ≈ 1,780,000 J/mol
ΔH ≈ 1,780 kJ/mol
So, the correct answer for the standard molar enthalpy of combustion for acetone is approximately -1780 kJ/mol.
Make sure to check your calculations and units to avoid any errors. It's also worth noting that the answer is negative (-1780 kJ/mol) because the combustion process is exothermic, releasing heat energy.