A 6.61 g sample of an unknown salt (MM = 116.82 g/mol) is dissolved in 150.00 g of water in a coffee cup calorimeter. Before placing the sample in the water, the temperature of the salt and water is 23.72 °C. After the salt has completely dissolved, the temperature of the solution is measured to be 20.15 °C. What is the enthalpy change (ΔH) for the dissolution of the salt in kJ/mol?

• A. -25.4 kJ/mol
• B. 25.4 kJ/mol
• C. -12.7 kJ/mol
• D. 12.7 kJ/mol

Answer: (A)

Verified step by step guidance

1. Calculate the mass of the solution by adding the mass of the water (150.00 g) and the mass of the salt (6.61 g).
2. Determine the temperature change (ΔT) of the solution by subtracting the final temperature (20.15 °C) from the initial temperature (23.72 °C).
3. Use the formula for heat transfer, q = m × c × ΔT, where m is the mass of the solution, c is the specific heat capacity of water (4.18 J/g°C), and ΔT is the temperature change, to calculate the heat absorbed or released by the solution.
4. Convert the heat absorbed or released (q) from joules to kilojoules by dividing by 1000.
5. Calculate the enthalpy change (ΔH) per mole of salt by dividing the heat change in kilojoules by the number of moles of salt, which is found by dividing the mass of the salt (6.61 g) by its molar mass (116.82 g/mol).