Final answer:
The work per unit mass done during the isothermal phase change from saturated vapor to saturated liquid in a piston-cylinder assembly is determined using the specific volume values for saturated vapor and liquid from steam tables and multiplying the difference by the saturation pressure at 200°C to get the value in kJ/kg.
Step-by-step explanation:
The question pertains to an isothermal process in a piston-cylinder assembly involving water undergoing a phase change from saturated vapor to saturated liquid at a constant temperature of 200°C. In this scenario, the work per unit mass can be calculated using the thermodynamic property tables for water. Since the process is isothermal and occurs at constant temperature, the water does not change its temperature, hence the internal energy does not change significantly during the phase change.
To determine the work done per unit mass, you would use the specific volume values for saturated vapor (vg) and saturated liquid (vf) at 200°C from the steam tables. The work done during this constant temperature process is obtained by the integral of PdV, which simplifies to P(vg - vf) as the pressure P remains constant during the phase change. Since this is a constant pressure process, multiplying the difference in specific volume by the saturation pressure at 200°C gives the specific work, which is then expressed in kJ/kg.