ANS:A - relative volatility is reasonably constant.

Fenske's equation is commonly used to estimate the minimum number of theoretical stages required for achieving a desired degree of separation in a binary distillation column. It is based on the assumption of constant relative volatility between the two components being separated. Let's explore why each of the options listed would affect the applicability of Fenske's equation:

1. Relative Volatility is Reasonably Constant:
   • Fenske's equation assumes that the relative volatility (the ratio of vapor pressures of the two components) remains constant throughout the distillation process.
   • When the relative volatility is reasonably constant, Fenske's equation provides a reliable estimation of the minimum number of theoretical stages needed for separation.
   • This condition is essential for the accurate application of Fenske's equation.
2. Mixture Shows Negative Deviation from Ideality:
   • Negative deviation from ideality implies that the vapor phase is richer in the more volatile component compared to what would be predicted by Raoult's law.
   • In such cases, the relative volatility may not remain constant throughout the distillation process.
   • Fenske's equation assumes ideal behavior and may not accurately predict the minimum number of stages required when negative deviations from ideality occur.
3. Mixture Shows Positive Deviation from Ideality:
   • Positive deviation from ideality implies that the vapor phase is poorer in the more volatile component compared to what would be predicted by Raoult's law.
   • Similar to negative deviation, positive deviations from ideality may cause fluctuations in the relative volatility during the distillation process.
   • Fenske's equation may not provide accurate results when positive deviations from ideality are present.
4. Multicomponent Distillation is Involved:
   • Fenske's equation is derived for binary systems and assumes a simple separation of two components.
   • In multicomponent distillation, interactions between more than two components may affect the relative volatilities and introduce complexities that are not accounted for in Fenske's equation.
   • While Fenske's equation may still provide a rough estimate, it may not accurately capture the intricacies of multicomponent separation processes.

In summary, Fenske's equation holds good when the relative volatility is reasonably constant, as it relies on this assumption for its accuracy. Deviations from ideality or involvement of multicomponent systems may introduce complexities that can affect the applicability of Fenske's equation.