GATE 2016 ME (ME1) – Question 55
For water at 25°C, $dp_s/dT_s=0.189$ kPa/K, where $p_s,T_s$ are saturation pressure and temperature. Dry saturated vapour specific volume is 43.38 m³/kg; liquid volume is negligible. Use Clausius–Clapeyron to estimate the enthalpy of evaporation at 25°C: ______ kJ/kg.
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Correct answer: 2400 to 2500
Explanation
The Clapeyron relation gives hfg=T(vg−vf)(dps/dT). Neglect vf as instructed and use T=298.15K, vg=43.38m³/kg and slope0.189kPa/K. Product is298.15·43.38·0.189=**2444.48kJ/kg** because kPa·m³ equals kJ.