Quantitative Analysis of Organic Compounds
Move from "what" is present to "how much" is present. Master the critical formulas of Liebig's, Dumas, Kjeldahl's, and Carius methods to secure your Physical-Organic numerical marks.
Module Focus: The Path to the Empirical Formula
Once qualitative analysis confirms the presence of specific elements, quantitative analysis determines their exact mass percentages. These percentages are the foundation for calculating the Empirical Formula. In NEET, you must memorize the reagents used to trap the elements and the mathematical formulas used to back-calculate their mass. Let $m$ always represent the total mass of the organic compound taken.
1. Estimation of Carbon & Hydrogen (Liebig's Method)
The organic compound is oxidized by heating with dry $CuO$. Carbon converts to $CO_2$ and Hydrogen to $H_2O$.
The $H_2O$ is trapped using anhydrous $CaCl_2$ or $Mg(ClO_4)_2$ (mass increases).
The $CO_2$ is trapped using concentrated $KOH$ solution (mass increases).
Since $44\text{g}$ of $CO_2$ contains $12\text{g}$ of Carbon:
Since $18\text{g}$ of $H_2O$ contains $2\text{g}$ of Hydrogen:
2. Estimation of Nitrogen
A. Dumas Method
The organic compound is heated with $CuO$ in an atmosphere of $CO_2$. Nitrogen is liberated as free $N_2$ gas. Any nitrogen oxides formed are reduced back to $N_2$ by passing over heated copper gauze. The $N_2$ gas is collected over an aqueous $KOH$ solution (which absorbs the $CO_2$ carrier gas).
Note: $V_{\text{STP}}$ is the volume of $N_2$ gas converted to STP (Standard Temp & Pressure) in mL.
$P_{dry} = P_{total} - \text{Aqueous Tension}$
B. Kjeldahl's Method
A highly important method, especially for fertilizers and foods. Instead of $N_2$ gas, Nitrogen is converted into Ammonia ($NH_3$).
Where:
M = Molarity of standard acid used to absorb $NH_3$
N = Normality of the acid ($N = M \times \text{Basicity}$)
V = Volume (in mL) of the acid neutralized by $NH_3$.
(Calculate V by taking total volume of acid taken minus the volume of acid left unreacted, determined by back-titration with NaOH).
Kjeldahl's method relies on converting organic nitrogen into $(NH_4)_2SO_4$. However, certain compounds are too stable or structured differently, and their nitrogen will NOT convert to ammonium sulfate under these conditions.
- Nitro compounds ($-NO_2$)
- Azo compounds ($-N=N-$)
- Nitrogen present in aromatic rings (e.g., Pyridine, Quinoline)
For these compounds, the Dumas method must be used.
3. Estimation of Halogens and Sulfur (Carius Method)
The organic compound is heated with fuming Nitric Acid ($HNO_3$) in a sealed, hard glass tube known as a Carius tube. Carbon and Hydrogen are oxidized to $CO_2$ and $H_2O$.
Silver Nitrate ($AgNO_3$) is added. The halogen forms a precipitate of Silver Halide ($AgX$), which is washed, dried, and weighed.
- $AgCl$ Molar Mass = 143.5 g/mol
- $AgBr$ Molar Mass = 188 g/mol
- $AgI$ Molar Mass = 235 g/mol
Sulfur is oxidized to Sulfuric acid ($H_2SO_4$). Barium Chloride ($BaCl_2$) is added to precipitate Barium Sulfate ($BaSO_4$).
Where 32 is atomic mass of S, and 233 is molar mass of $BaSO_4$.
4. Estimation of Phosphorus and Oxygen
Heated with fuming $HNO_3$ to oxidize P to Phosphoric acid ($H_3PO_4$). It is precipitated as magnesium ammonium phosphate, which is then ignited to yield Magnesium Pyrophosphate ($Mg_2P_2O_7$).
Note: 62 is the mass of 2 Phosphorus atoms, 222 is the molar mass of $Mg_2P_2O_7$.
Usually, Oxygen is estimated by difference: $\%O = 100 - (\%C + \%H + \%N...)$
However, in a direct method, the compound is decomposed to release $O_2$, which is passed over hot carbon to form $CO$. The $CO$ is then oxidized by Iodine Pentoxide ($I_2O_5$) into $CO_2$ while liberating $I_2$.
NEET Grand Test: Quantitative Analysis
15 High-Yield Questions targeting numerical formulas, standard precipitates, and Kjeldahl limitations.
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