Chemistry - Thermodynamics - I Question with Solution | TestHub
The vapour pressure of water at 343 K is 234 mm Hg. The external pressure on H₂O (l) taken in a cylinder fitted with a frictionless movable piston initially containing 1.8 L (= 1.8 kg) of H₂O (l) at 343 K is increased to 1 atm at constant temperature. Then heat is supplied keeping the pressure constant till 0.9 L of H₂O (l) is evaporated to form H₂O (g) at 373 K. Assume the internal energy of the liquid to be dependent only on temperature. Answer the following questions by carefully observing the diagrams and the data provided. Given:
Specific heat of H₂O (l) = 4.2 J/°C-g
ΔHvap at 373 K and 1 atm = +40 kJ/mol
1 L-atm = 100 J
R = 8 J/K-mol
ΔH (in J), when system is taken from State 1 to State 2, is
Options:
Answer:
Solution:
State 1 → 2 is isothermal for the liquid, so ΔU = 0 and ΔH = Δ(PV) = V(P₂ − P₁) with the liquid volume constant at 1.8 L: ΔH = 1.8 × (760 − 234)/760 L-atm = 1.8 × 526/760 ≈ 1.246 L-atm ≈ 124.6 J. Matches (b).
