To quantify the heat evolved when potassium hydroxide dissolves in water, a student performs three calorimetry trials using a coffee-cup calorimeter. In each trial, solid KOH is quickly added to 100.0 g of water initially at 21.0 °C. The maximum temperature reached is recorded. The data are summarized below.
Trial | mass KOH (g) | final T (°C)
1 | 2.80 ± 0.05 | 25.6 ± 0.1
2 | 3.10 ± 0.05 | 26.1 ± 0.1
3 | 2.95 ± 0.05 | 25.8 ± 0.1
Assume:
• density of the solution = 1.00 g mL−1
• specific heat capacity of the solution = 4.18 J g−1∘C−1 (negligible uncertainty)
• molar mass of KOH = 56.11 g mol−1
• the calorimeter absorbs no heat.
Answer the following. Express all heats in kilojoules and all enthalpy changes in kJ mol−1.
(a) For each trial, calculate the heat, q, absorbed by the solution. Include the correct sign in each value.
(b) Using the measured masses of KOH, determine the molar enthalpy change, ΔH, for each trial.
(c) Propagate the stated measurement uncertainties to determine the absolute uncertainty in the ΔH value for Trial 2. Clearly show how each uncertainty contributes to the final result.
(d) The literature value for the dissolution of KOH under standard conditions is −56.0kJ mol−1. Using the average of the three trials and the uncertainty calculated in part (c), assess whether the experimental value is consistent with the literature value. Then, identify and briefly discuss two plausible systematic sources of error that could explain any discrepancy.