Estimation of Carbon, Hydrogen, Nitrogen, Sulphur, Phosphorus — Revision Notes

⚡ 30-Second Revision

Carbon & Hydrogen (Liebig's): — Burn compound $\rightarrow$ CO$_2$ (absorb in KOH), H$_2$O (absorb in CaCl$_2$). \n $\text{\%C} = \frac{12}{44} \times \frac{\text{mass of CO}_2}{\text{mass of compound}} \times 100$ \n $\text{\%H} = \frac{2}{18} \times \frac{\text{mass of H}_2\text{O}}{\text{mass of compound}} \times 100$ \n- Nitrogen (Dumas): Burn compound with CuO $\rightarrow$ N$_2$ gas. Collect N$_2$ over KOH. Correct volume to STP. \n $\text{V}_{STP} = V \times \frac{273}{T} \times \frac{P - P_{aq}}{760}$ \n $\text{\%N} = \frac{28}{22400} \times \frac{\text{V}_{STP}}{\text{mass of compound}} \times 100$ \n- Nitrogen (Kjeldahl): Digest with H$_2$SO$_4$ $\rightarrow$ (NH$_4$)$_2$SO$_4$. Distill NH$_3$ into std. acid. Back titrate. \n $\text{\%N} = \frac{(\text{V}_{acid} \times \text{N}_{acid}) - (\text{V}_{base} \times \text{N}_{base})}{1000 \times \text{mass of compound}} \times 14 \times 100$ \n *Limitations:* Not for nitro, azo, or ring N. \n- Sulphur (Carius): Heat with fuming HNO$_3$ $\rightarrow$ H$_2$SO$_4$. Precipitate as BaSO$_4$. \n $\text{\%S} = \frac{32}{233} \times \frac{\text{mass of BaSO}_4}{\text{mass of compound}} \times 100$ \n- Phosphorus (Carius): Heat with fuming HNO$_3$ $\rightarrow$ H$_3$PO$_4$. Precipitate as MgNH$_4$PO$_4$, ignite to Mg$_2$P$_2$O$_7$. \n $\text{\%P} = \frac{62}{222} \times \frac{\text{mass of Mg}_2\text{P}_2\text{O}_7}{\text{mass of compound}} \times 100$

2-Minute Revision

Quantitative estimation of elements is crucial for determining the composition of organic compounds. For carbon and hydrogen, Liebig's method involves complete combustion of a known mass of the compound to form CO$_2$ and H$_2$O.

These products are absorbed by specific reagents (anhydrous CaCl$_2$ for H$_2$O, KOH for CO$_2$), and their increased masses are used to calculate the percentages of C and H. \n\nNitrogen can be estimated by two main methods: Dumas and Kjeldahl.

The Dumas method is universal; it converts all nitrogen to N$_2$ gas, whose volume is measured and corrected to STP for calculation. Kjeldahl's method is for compounds where nitrogen can be converted to ammonium sulphate; it involves digestion, distillation of ammonia, and titration.

Remember, Kjeldahl's is not for nitro, azo, or ring nitrogen compounds. \n\nSulphur and phosphorus are typically estimated by the Carius method. The compound is heated with fuming nitric acid in a sealed tube.

Sulphur is oxidized to H$_2$SO$_4$, which is then precipitated and weighed as BaSO$_4$. Phosphorus is oxidized to H$_3$PO$_4$, which is eventually weighed as magnesium pyrophosphate (Mg$_2$P$_2$O$_7$).

Mastering the specific formulas and their stoichiometric factors for each element is key for NEET.

5-Minute Revision

For NEET, a solid grasp of elemental estimation methods is non-negotiable. Start with Carbon and Hydrogen (Liebig's Method). The principle is combustion: C $\rightarrow$ CO$_2$, H $\rightarrow$ H$_2$O.

Remember the absorbents: anhydrous CaCl$_2$ for H$_2$O and KOH for CO$_2$. The key formulas are: \n$\text{\%C} = \frac{12}{44} \times \frac{\text{mass of CO}_2}{\text{mass of compound}} \times 100$ and $\text{\%H} = \frac{2}{18} \times \frac{\text{mass of H}_2\text{O}}{\text{mass of compound}} \times 100$.

\n\nNext, Nitrogen estimation has two methods. Dumas Method is universal. Nitrogen is converted to N$_2$ gas, collected over KOH (which absorbs CO$_2$), and its volume is measured. Crucially, convert this volume to STP using $V_{STP} = V \times \frac{273}{T} \times \frac{P - P_{aq}}{760}$.

Then, $\text{\%N} = \frac{28}{22400} \times \frac{\text{V}_{STP}}{\text{mass of compound}} \times 100$. \nKjeldahl's Method is for specific nitrogen compounds (not nitro, azo, or ring N). It involves digestion with conc.

H$_2$SO$_4$ to form (NH$_4$)$_2$SO$_4$, followed by distillation of NH$_3$ into a known excess of standard acid, and then back-titration. The formula is $\text{\%N} = \frac{(\text{V}_{acid} \times \text{N}_{acid}) - (\text{V}_{base} \times \text{N}_{base})}{1000 \times \text{mass of compound}} \times 14 \times 100$.

Pay attention to milliequivalents. \n\nFor Sulphur and Phosphorus (Carius Method), the compound is heated with fuming HNO$_3$. For sulphur, it forms H$_2$SO$_4$, which is precipitated as BaSO$_4$.

\n$\text{\%S} = \frac{32}{233} \times \frac{\text{mass of BaSO}_4}{\text{mass of compound}} \times 100$. \nFor phosphorus, it forms H$_3$PO$_4$, which is eventually weighed as Mg$_2$P$_2$O$_7$. \n$\text{\%P} = \frac{62}{222} \times \frac{\text{mass of Mg}_2\text{P}_2\text{O}_7}{\text{mass of compound}} \times 100$.

\n\nWorked Example (Kjeldahl): 0.2 g compound, NH$_3$ absorbed in 20 mL 0.1 N H$_2$SO$_4$. Excess acid needed 10 mL 0.1 N NaOH. \nMilliequivalents of H$_2$SO$_4$ reacted with NH$_3$ = $(20 \times 0.

1) - (10 \times 0.1) = 2 - 1 = 1$. \nMass of N =$1 \times 14, ext{mg} = 0.014, ext{g}$. \n$\text{\%N} = \frac{0.014}{0.

Prelims Revision Notes

Elemental Estimation: Key Facts for NEET\n\n1. Carbon and Hydrogen (Liebig's Method):\n* Principle: Complete combustion of organic compound in excess O$_2$ over CuO.\n* Products: Carbon $\rightarrow$ CO$_2$, Hydrogen $\rightarrow$ H$_2$O.\n* Absorption: H$_2$O absorbed by anhydrous CaCl$_2$ (or Mg(ClO$_4$)$_2$). CO$_2$ absorbed by concentrated KOH solution.\n* Order of Absorption: H$_2$O first, then CO$_2$. (KOH absorbs both, CaCl$_2$ only H$_2$O).\n* Formulas:\n * $\text{\%C} = \frac{12}{44} \times \frac{\text{Mass of CO}_2}{\text{Mass of organic compound}} \times 100$\%\n * $\text{\%H} = \frac{2}{18} \times \frac{\text{Mass of H}_2\text{O}}{\text{Mass of organic compound}} \times 100$\%\n\n2. Nitrogen Estimation:\n A. Dumas Method:\n * Principle: Organic compound heated with CuO in CO$_2$ atmosphere. Nitrogen $\rightarrow$ N$_2$ gas.\n * Reduction: Oxides of nitrogen (if formed) reduced to N$_2$ by hot copper gauze.\n * Collection: N$_2$ collected over aqueous KOH solution (absorbs CO$_2$).\n * Volume Correction: Convert observed N$_2$ volume to STP using: \n $\text{V}_{STP} = V \times \frac{273}{T} \times \frac{P - P_{aq}}{760}$ (T in K, P in mmHg)\n * Formula: $\text{\%N} = \frac{28}{22400} \times \frac{\text{V}_{STP}}{\text{Mass of organic compound}} \times 100$\%\n * Applicability: Universal for all N-containing organic compounds.\n\n B. Kjeldahl's Method:\n * Principle: Nitrogen $\rightarrow$ (NH$_4$)$_2$SO$_4$ (digestion with conc. H$_2$SO$_4$ + catalyst). (NH$_4$)$_2$SO$_4$ $\rightarrow$ NH$_3$ (with NaOH). NH$_3$ absorbed in excess standard acid, then back-titrated.\n * Catalysts: CuSO$_4$ (accelerates reaction), K$_2$SO$_4$ (raises boiling point of H$_2$SO$_4$).\n * Limitations: NOT for compounds with N in nitro ($-$NO$_2$), azo ($-$N=N$-$), or heterocyclic rings (e.g., pyridine, quinoline) as they don't convert to (NH$_4$)$_2$SO$_4$ quantitatively.\n * Formula: $\text{\%N} = \frac{(\text{V}_{acid} \times \text{N}_{acid}) - (\text{V}_{base} \times \text{N}_{base})}{1000 \times \text{Mass of organic compound}} \times 14 \times 100$\%\n (Where 14 is atomic mass of N, 1000 converts mg to g)\n\n3. Sulphur (Carius Method):\n* Principle: Organic compound heated with fuming HNO$_3$ in sealed tube. Sulphur $\rightarrow$ H$_2$SO$_4$.\n* Precipitation: H$_2$SO$_4$ precipitated as BaSO$_4$ by adding BaCl$_2$.\n* Formula: $\text{\%S} = \frac{32}{233} \times \frac{\text{Mass of BaSO}_4}{\text{Mass of organic compound}} \times 100$\%\n (Atomic mass S = 32, Molecular mass BaSO$_4$ = 233)\n\n4. Phosphorus (Carius Method):\n* Principle: Organic compound heated with fuming HNO$_3$ in sealed tube. Phosphorus $\rightarrow$ H$_3$PO$_4$.\n* Precipitation: H$_3$PO$_4$ precipitated as MgNH$_4$PO$_4$ (using magnesia mixture), then ignited to Mg$_2$P$_2$O$_7$.\n* Formula: $\text{\%P} = \frac{62}{222} \times \frac{\text{Mass of Mg}_2\text{P}_2\text{O}_7}{\text{Mass of organic compound}} \times 100$\%\n (Mass of 2P = 62, Molecular mass Mg$_2$P$_2$O$_7$ = 222)\n\nGeneral Tips: Pay attention to atomic/molecular masses, unit conversions (mL to L, mg to g), and significant figures. Practice numerical problems extensively.

Vyyuha Quick Recall

To remember the elements and their estimation methods, think: 'C H N S P - L D K C C'\n\n* C H: Liebig's (Combustion)\n* N: Dumas (Gasometric), Kjeldahl (Titrimetric)\n* S: Carius (BaSO$_4$ ppt)\n* P: Carius (Mg$_2$P$_2$O$_7$ ppt)\n\nThis helps recall the primary method for each element. For Kjeldahl's limitation, remember: 'No Nitro, No Azo, No Ring N for Kjeldahl!'