Questions: 2. Calcium oxide reacts with water to produce calcium hydroxide and 65.2 kJ of heat in the following reaction. CaO(s) + H2O(l) -> Ca(OH)2(s) ΔH = -65.2 kJ / mol a. How much heat is released when 78.0 g of calcium oxide reacts with excess water?

Solution Steps

The molar mass of calcium oxide (CaO) is calculated as follows: $$\text{Molar mass of CaO} = \text{Molar mass of Ca} + \text{Molar mass of O}$$ $$\text{Molar mass of Ca} = 40.08 \, \text{g/mol}$$ $$\text{Molar mass of O} = 16.00 \, \text{g/mol}$$ $$\text{Molar mass of CaO} = 40.08 \, \text{g/mol} + 16.00 \, \text{g/mol} = 56.08 \, \text{g/mol}$$

Given the mass of calcium oxide is 78.0 g, we can calculate the number of moles using the molar mass: $$\text{Number of moles of CaO} = \frac{\text{Mass of CaO}}{\text{Molar mass of CaO}}$$ $$\text{Number of moles of CaO} = \frac{78.0 \, \text{g}}{56.08 \, \text{g/mol}} = 1.390 \, \text{mol}$$

The reaction releases 65.2 kJ of heat per mole of calcium oxide. Therefore, the total heat released can be calculated as: $$\text{Heat released} = \text{Number of moles of CaO} \times \Delta H$$ $$\text{Heat released} = 1.390 \, \text{mol} \times (-65.2 \, \text{kJ/mol}) = -90.67 \, \text{kJ}$$

Final Answer