A 1.00-g sample of solid calcium was burned in fluorine to give a 1.95 g sample of pure CaF2. The energy given off in this reaction was 30.3 kJ. Given this information, we can say that the heat of formation of 1 mol of CaF2 is
Heat formation CaF2 is 30.3 kJ/1.95 g CaF2. Convert 1.95 g to moles if you want the answer in kJ/mol which is the usual way you find it in table.
To calculate the heat of formation of 1 mole of CaF2, we need to first determine the moles of CaF2 produced in the reaction.
1. Calculate the moles of calcium (Ca) used:
- Given mass of calcium = 1.00 g
- Molar mass of Ca = 40.08 g/mol (from periodic table)
Moles of Ca = mass of Ca / molar mass of Ca
= 1.00 g / 40.08 g/mol
≈ 0.02495 mol (approximately 0.025 mol)
2. Calculate the moles of CaF2 produced:
- Given mass of CaF2 = 1.95 g
- Molar mass of CaF2 = 40.08 g/mol (for Ca) + 37.99 g/mol (for F)
Moles of CaF2 = mass of CaF2 / molar mass of CaF2
= 1.95 g / (40.08 g/mol + 37.99 g/mol)
= 1.95 g / 78.07 g/mol
≈ 0.02497 mol (approximately 0.025 mol)
3. Calculate the heat of formation:
The heat of formation (ΔHf) can be calculated using the formula:
ΔHf = q / n
where
ΔHf = heat of formation
q = energy given off in the reaction (30.3 kJ, but convert to J)
n = moles of the substance (CaF2)
ΔHf = 30.3 kJ / 0.025 mol
= 30,300 J / 0.025 mol
= 1,212,000 J/mol
= 1.212 MJ/mol
So, we can say that the heat of formation of 1 mole of CaF2 is approximately 1.212 MJ/mol.