Chemistry - Thermodynamics - I Question with Solution | TestHub
The vapour pressure of water at 353 K is 532 mm Hg. The external pressure on H₂O (l) taken in a cylinder fitted with frictionless movable piston initially containing 0.9 L (= 0.9 kg) of H₂O (l) at 353 K is increased to 1 atm at the constant temperature. Then, heat is supplied keeping the pressure constant till 0.45 L of H₂O (l) is evaporated to form H₂O (g) at 373 K. Assume the internal energy of 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 an isothermal pressure change of the liquid only. Since U of the liquid depends only on T, ΔU = 0. ΔH = ΔU + Δ(PV) = V(P₂ − P₁) (liquid volume constant at 0.9 L) = 0.9 L × (760 − 532)/760 atm = 0.9 × 0.3 = 0.27 L-atm = 0.27 × 100 = 27 J. Matches (c).
