ΔH When system is taken from state 1 to state 2 (Joule)? (A) Zero (B) 0.27 (C) 27 (D) 90

Question

The vapor pressure of H₂O (l) 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 33 K is increased to 1 atm. Temperature remained constant. Now, heat is supplied keeping pressure constant till 0.45 L of H₂O (l) (=0.45 kg) is evaporated to form H₂O (g) at 373 K. carefully observe the diagrams provided and form given data, answer the following questions Given:

Specific heat of H₂O = 4.2J/gm⁰C

Use H_vap at 373 K and 1 atm =+40 kJ/mol

1L atm = 100 Joule

1 atm = 760 mm Hg

R = 8 Joule/mole K 

(Assume internal energy of liquid to be dependent only on temperature).

ΔH When system is taken from state 1 to state 2 (Joule)? 

(A) Zero 

(B) 0.27 

(C) 27 

(D) 90 

Answer 1

Correct option (C) 27

Explanation:

(i) T = 532 mm Hg = 0.4 atm T = 353 k

V_i = 0.4 = 0.4 kg

∆H = ∆U + ∆_pV

Dependent only on temperature

∴ ∆H = ∆_pV = (1–0.7) × 0.9 L

= 0.3 × 0.9 L = 0.3 × 0.9 × 100 J= 27 J