By-product route — Ammonium sulfate [(NH4)2SO4] can be produced industrially by reacting gypsum (CaSO4·2H2O) with which reagent under appropriate conditions?

Introduction:
Ammonium sulfate is manufactured both as a primary product and as a by-product in caprolactam and coke-oven processes. One classical route uses natural gypsum as a sulfur source, forming ammonium sulfate while precipitating calcium carbonate—an attractive utilization of mineral by-products.

Given Data / Assumptions:

• Starting solid: gypsum (CaSO4·2H2O).
• Goal: produce (NH4)2SO4 in aqueous media.
• Carbonate availability via ammonium carbonate or via NH3 + CO2 + H2O generating (NH4)2CO3 in situ.

Concept / Approach:

The net reaction proceeds effectively when ammonium carbonate reacts with gypsum to yield ammonium sulfate and calcium carbonate: CaSO4·2H2O + (NH4)2CO3 → (NH4)2SO4 + CaCO3 + 2H2O. In practice, NH3 and CO2 are sparged into a gypsum slurry to form ammonium carbonate in situ, which then drives the conversion. Direct reaction with NH3 alone is insufficient; nitric acid would produce calcium nitrate rather than target ammonium sulfate.

Step-by-Step Solution:

Verification / Alternative check:

Process descriptions for gypsum-to-ammonium-sulfate plants detail carbonation and ammoniation steps leading to CaCO3 precipitation and ammonium sulfate crystallization.

Why Other Options Are Wrong:

A: NH3 alone does not displace sulfate from CaSO4 effectively. C: Nitric acid would form Ca(NO3)2. D: Not applicable because a suitable reagent exists. E: Ammonium chloride would form CaCl2, not the desired product.

Common Pitfalls:

Forgetting the role of carbonate in precipitating CaCO3, which shifts equilibrium toward ammonium sulfate formation.

Final Answer:

ammonium carbonate