Answer to Question #256744 in Chemistry for Faye

Question

Calculate the enthalpy change when 14.5 g of water vapor condenses to liquid water.

Answer 1

Solution:

The molar mass of H₂O is 18.015 g/mol

Calculate the moles of water:

(14.5 g H₂O) × (1 mol H₂O / 18.015 g H₂O) = 0.805 mol H₂O

The enthalpy change (ΔH) associated with converting a gas into liquid is known as the heat of condensation (ΔH_cond).

The molar heat of condensation ΔH_cond of a substance is the heat released by one mole of that substance as it is converted from a gas to a liquid. 

ΔH_cond = −ΔH_vap

The molar heat of vaporization (ΔH_vap) of H₂O is 40.7 kJ/mol

Hence, ΔH_cond = −ΔH_vap = −40.7 kJ/mol

Calculate the enthalpy change (ΔH):

(0.805 mol H₂O) × (−40.7 kJ / 1 mol H₂O) = −32.76 kJ

ΔH = −32.76 kJ

Answer: The enthalpy change (ΔH) is −32.76 kJ