Quantitative Analysis in Organic Chemistry

Let’s explore how chemists measure elements in compounds! 🔍

🔥 Carbon & Hydrogen Estimation

Burn organic compound with excess oxygen & CuO:

\ce{C_xH_y + (x + y/4)O2 -> x CO2 + (y/2) H2O}

Apparatus setup:
Organic compound → Combustion tube → U-tube (anhydrous CaCl₂ for H₂O) → U-tube (KOH for CO₂)

Calculations:

  • Mass of compound = m g
  • Mass of CO₂ produced = m₂ g
  • Mass of H₂O produced = m₁ g
\% \text{C} = \frac{12 \times m_2 \times 100}{44 \times m} \\ \% \text{H} = \frac{2 \times m_1 \times 100}{18 \times m}

Example: 0.246g compound → 0.198g CO₂ & 0.1014g H₂O
%C = 21.95%, %H = 4.58% ✅

⛓ Nitrogen Estimation

Method 1: Dumas Method
Heat compound with CuO in CO₂ atmosphere → Measure N₂ gas

\text{Volume at STP} = \frac{P_1V_1 \times 273}{760 \times T_1} \\ \% \text{N} = \frac{28 \times V_{\text{STP}} \times 100}{22400 \times m}

Example: 0.3g compound → 50mL N₂ at 300K & 700mm pressure → %N = 17.46%

Method 2: Kjeldahl Method
For -NH₂ and -CONH₂ groups only!
Compound + conc. H₂SO₄ → (NH₄)₂SO₄ → NH₃ gas absorbed in acid

\% \text{N} = \frac{1.4 \times M \times 2(V – V_1/2)}{m}

Where M = acid molarity, V = acid vol, V₁ = NaOH vol for titration
Example: 0.5g compound → 10mL 1M H₂SO₄ neutralized → %N = 56% 🧪

🧪 Halogen Estimation (Carius Method)

Heat compound with fuming HNO₃ + AgNO₃ → Precipitate AgX

\% \text{X} = \frac{\text{Atomic mass of X} \times m_1 \times 100}{\text{Mol. mass of AgX} \times m}

Example: 0.15g compound → 0.12g AgBr → %Br = 34.04%

💛 Sulphur Estimation

Compound + Na₂O₂/fuming HNO₃ → H₂SO₄ → BaSO₄ precipitate

\% \text{S} = \frac{32 \times m_1 \times 100}{233 \times m}

Example: 0.157g compound → 0.4813g BaSO₄ → %S = 42.10%

📌 Phosphorus Estimation

Two methods:

  1. Precipitate as (NH₄)₃PO₄·12MoO₃:
    \% \text{P} = \frac{31 \times m_1 \times 100}{1877 \times m}
  2. Precipitate as Mg₂P₂O₇:
    \% \text{P} = \frac{62 \times m_1 \times 100}{222 \times m}

💨 Oxygen Estimation

%O = 100% – (sum of all other elements)
OR Direct method:

\% \text{O} = \frac{32 \times m_1 \times 100}{88 \times m}

(where m₁ = mass of CO₂ produced from oxygen conversion)

⭐ NEET Super Shorts

  • 🧪 Kjeldahl limitation: Doesn’t work for nitro (-NO₂), azo (-N=N-), or ring nitrogen (e.g., pyridine)
  • ⚖️ Dumas vs Kjeldahl: Dumas for all N-compounds, Kjeldahl only for amino/proteins
  • 💡 Sulphur test trick: Violet color with sodium nitroprusside = S present!
  • 🔥 Combustion analysis: CaCl₂ absorbs H₂O, KOH absorbs CO₂
  • 💧 Halogen test: Always decompose cyanides/sulphides first if N/S present!

Happy studying! 💯