Question

calculating heat during phase changes
the molar heat of vaporization of hydrogen fluoride (hf) is 31.18 kj/mol. how much heat is released when 105.5 g hf condenses at its boiling point? use the formula ( q = ndelta h ).
options: 155.8 kj, 160.2 kj, -155.8 kj, -160.2 kj

Explanation:

Step1: Calculate moles of HF

Molar mass of HF: H (1.008 g/mol) + F (19.00 g/mol) = 20.008 g/mol.
Moles \( n = \frac{\text{mass}}{\text{molar mass}} = \frac{106.5\ \text{g}}{20.008\ \text{g/mol}} \approx 5.323\ \text{mol} \).

Step2: Calculate heat using \( q = n\Delta H \)

Given \( \Delta H = 31.18\ \text{kJ/mol} \),
\( q = 5.323\ \text{mol} \times 31.18\ \text{kJ/mol} \approx 165.8\ \text{kJ} \).

Answer:

165.8 kJ