Gas solubility in water (qualitative): Among NH3, CO2, H2S, and CH4, which gas is the most soluble in liquid water under comparable conditions?

Introduction / Context:
Gas solubility in water is central to scrubbing, wastewater aeration, absorption design, and environmental fate of gaseous pollutants. Chemical interactions with water, polarity, and acid–base behavior can dramatically enhance apparent solubility beyond simple physical dissolution governed by Henry’s law constants alone.

Given Data / Assumptions:

• Gases compared: ammonia (NH3), carbon dioxide (CO2), hydrogen sulfide (H2S), methane (CH4).
• Comparable temperature and pressure; dilute solutions; no added electrolytes.
• Focus on relative solubility in pure water.

Concept / Approach:
NH3 is highly soluble due to its ability to hydrogen-bond and, importantly, react with water to form NH4+ and OH− (base hydrolysis), effectively consuming dissolved NH3 and increasing total ammonia species in solution. CO2 is also reactive, forming carbonic acid/bicarbonate, giving it moderate solubility. H2S exhibits weak acidity and moderate solubility. CH4 is nonpolar and exhibits very low aqueous solubility governed mostly by weak dispersion interactions, making it the least soluble of the set.

Step-by-Step Solution:

Verification / Alternative check:
Henry’s law constants and published solubility data rank NH3 as highly soluble (tens of % w/w possible under pressure), CO2 and H2S as moderately soluble, and CH4 as poorly soluble at ambient conditions.

Why Other Options Are Wrong:

• CO2, H2S: more soluble than CH4 but significantly less than NH3 due to weaker or less favorable reactions.
• CH4: least soluble; hydrophobic, non-reactive.
• N2: included as an extra distractor; also low solubility.

Common Pitfalls:
Comparing only physical solubility constants without accounting for acid–base reactions; assuming all gases behave inertly in water.

Final Answer:
NH3