In gas absorption, for which solute absorbed in water is the overall mass transfer coefficient effectively based only on the gas-film coefficient because the gas-side resistance controls?

Introduction / Context:
Overall mass transfer during gas absorption depends on resistances in both the gas and liquid films. For highly soluble gases, the liquid-side resistance becomes negligible, leaving the gas-side resistance controlling, so the overall coefficient reflects the gas film coefficient predominantly.

Given Data / Assumptions:

• Absorption into liquid water at typical process conditions.
• Comparative solubility: NH3 is highly soluble; CO2 and SO2 are moderately soluble with notable liquid-phase equilibria.
• Film theory: total resistance = gas film + interface + liquid film; dominance depends on solubility and kinetics.

Concept / Approach:
For very soluble gases, the equilibrium partial pressure at the interface is small, reducing the driving force loss in the liquid phase. Hence, the liquid-film resistance is negligible and the gas-film controls. Ammonia (NH3) dissolves readily in water and reacts to form ammonium hydroxide species, further enhancing apparent solubility and reducing liquid-side resistance.

Step-by-Step Solution:
Step 1: Rank gases by solubility in water: NH3 >> SO2 ≈ CO2 (context-dependent), H2S varies but less than NH3.Step 2: Identify controlling resistance: highly soluble solutes → gas-film control.Step 3: Conclude that overall K is effectively based on gas-film coefficient for NH3 in water.Step 4: Select NH3 as the correct option.

Verification / Alternative check:
Design heuristics and classic absorption examples consistently treat NH3–water systems as gas-film controlling in packed towers at moderate conditions, aligning with observed high apparent Henry’s law departure due to chemical interaction.

Why Other Options Are Wrong:
CO2: Often shows appreciable liquid-film effects and equilibria; not purely gas-film controlled.SO2: Reactive/soluble but commonly not treated as purely gas-film controlled in generic water systems.None of these: Incorrect because NH3 fits.H2S: Soluble, but not typically the standard textbook example for purely gas-film control like NH3.

Common Pitfalls:
Assuming all acidic gases are gas-film controlled; actual control depends on solubility, reaction, and operating conditions. Always verify with equilibrium and rate data.

Final Answer:
NH3