Environmental Chemistry & Stoichiometry

Quick Answer: Convert mg/L → mol/L, balance equations, identify limiting reagent, and express results on per-equivalent basis for alkalinity/hardness.

Unit Conversions

[ \text{mol/L} = \frac{\text{mg/L}}{\text{molar mass (mg/mol)}} ]

Example: 50 mg/L NO₃⁻-N (as N) vs nitrate mass — read whether stem uses as N or as NO₃.

Balancing Reactions

Chlorination: Cl₂ + H₂O → HOCl + H⁺ + Cl⁻. Lime softening removes hardness as CaCO₃ precipitate.

Limiting Reagent

If 100 mg/L Ca²⁺ and 80 mg/L alkalinity available, precipitation limited by whichever stoichiometry exhausts first.

Alkalinity Equivalents

Alkalinity often reported as CaCO₃ mg/L. 1 meq/L = 50 mg/L as CaCO₃.

Exam Traps

• Confusing mg/L as element vs as compound.
• Forgetting density ≈ 1 for dilute water when converting ppm.

Multi-Step Workflow

List givens with units, select the governing relationship, convert to a consistent unit set, solve, and compare to a rough estimate.

Stoichiometry for Environmental Engineers

Worked Stoichiometry

Chlorine dose to oxidize ammonia (simplified breakpoint idea): mass ratios are tested conceptually. If 10 mg/L NH₃-N requires roughly 7.6× Cl₂ by weight at ideal breakpoint, a stem may ask order-of-magnitude dose — compute $10\times7.6=76$ mg/L Cl₂ before residual free chlorine appears.

Alkalinity as CaCO₃: $50$ mg/L as CaCO₃ $= 1$ meq/L. Softening and coagulation consume alkalinity — expect questions linking lime dose to alkalinity change.

On the Exam: Always convert mg/L ↔ mmol/L with molar mass before applying reaction ratios.

Stoichiometry for Environmental Engineers

Worked Stoichiometry

Chlorine dose to oxidize ammonia (simplified breakpoint idea): mass ratios are tested conceptually. If 10 mg/L NH₃-N requires roughly 7.6× Cl₂ by weight at ideal breakpoint, a stem may ask order-of-magnitude dose — compute $10\times7.6=76$ mg/L Cl₂ before residual free chlorine appears.

Alkalinity as CaCO₃: $50$ mg/L as CaCO₃ $= 1$ meq/L. Softening and coagulation consume alkalinity — expect questions linking lime dose to alkalinity change.

On the Exam: Always convert mg/L ↔ mmol/L with molar mass before applying reaction ratios.

Stoichiometry for Environmental Engineers

Worked Stoichiometry

Chlorine dose to oxidize ammonia (simplified breakpoint idea): mass ratios are tested conceptually. If 10 mg/L NH₃-N requires roughly 7.6× Cl₂ by weight at ideal breakpoint, a stem may ask order-of-magnitude dose — compute $10\times7.6=76$ mg/L Cl₂ before residual free chlorine appears.

Alkalinity as CaCO₃: $50$ mg/L as CaCO₃ $= 1$ meq/L. Softening and coagulation consume alkalinity — expect questions linking lime dose to alkalinity change.

On the Exam: Always convert mg/L ↔ mmol/L with molar mass before applying reaction ratios.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.