The question
Name: ______________________ Date: ____________ Why ammonium chloride dissolves (10 points)
Data table
Dissolving ammonium chloride in a calorimeter
A student adds 5.35 g of NH₄Cl(s) (molar mass 53.49 g/mol) to 50.00 g of water in a polystyrene-cup calorimeter, stirs until it dissolves completely, and records the temperature. Assume the solution has a specific heat capacity of 4.18 J/(g·°C) and that no heat is exchanged with the cup or the room.
NH₄Cl(s) → NH₄⁺(aq) + Cl⁻(aq); ΔS°soln = +75.0 J/(mol·K). R = 8.314 J/(mol·K).
| Measurement | Value |
|---|---|
| Mass of NH₄Cl | 5.35 g |
| Mass of water | 50.00 g |
| Initial temperature | 23.50 °C |
| Lowest temperature reached | 17.10 °C |
(a) Calculate the magnitude of the heat, in joules, absorbed from the solution as the NH₄Cl dissolves. [1 point]
(b) Calculate ΔH°soln for NH₄Cl, in kJ/mol. Include the sign. [2 points]
(c) Justify the positive sign of ΔS°soln in terms of the particles involved. [1 point]
(d) Use ΔH°soln = +14.8 kJ/mol. Calculate ΔG°soln at 298 K. [2 points]
(e) Dissolving NH₄Cl absorbs heat, yet it dissolves on its own at 298 K. Explain why, using your answers to parts (c) and (d). [1 point]
(f) Calculate the value of the equilibrium constant for NH₄Cl(s) ⇌ NH₄⁺(aq) + Cl⁻(aq) at 298 K. [1 point]
(g) Some of the NH₄Cl stuck to the weighing paper and never entered the calorimeter, but the student used 5.35 g in the calculation. Determine whether the student’s calculated ΔH°soln is too large, too small or unaffected. Justify your answer. [1 point]
(h) The student heats the water to 50 °C before repeating the experiment. Predict whether more or less NH₄Cl can dissolve, and justify your prediction using ΔG°. [1 point]