Chemistry - Thermochemistry Question with Solution | TestHub
A solution was prepared by dissolving 7.45 g of KCl in 200 g of H2O in a calorimeter with a water equivalent of 25 g. The temperature of the water had reduced from 28°C to 25°C. ΔH (in kJ/mol) for dissolving KCl in water is (Specific heat capacity of water is 4.2 J/K-g.)
Options:
Answer:
Solution:
First, calculate the moles of KCl:
nKCl = 7.45 g/(74.5 g/mol) = 0.1 mol
Next, calculate the heat absorbed by the water and calorimeter. The total mass is the sum of the water and calorimeter masses, and the temperature change is 3 °C.
Q = (mwater + mcalorimeter) × c × Δ T
Q = (200 g + 25 g) × 4.2 J/(g·°C) × 3 °C = 2835 J
Now, determine the heat absorbed per mole of KCl:
Δ Hper mole = 2835 J/0.1 mol = 28350 J/mol = 28.35 kJ/mol
Since the temperature fell, the process is endothermic, meaning heat was absorbed from the surroundings. Therefore, the enthalpy change (Δ H) is positive:
Δ H = +28.35 kJ/mol
This corresponds to option (d).
