Qualitative and Quantitative Analysis — Revision Notes

⚡ 30-Second Revision

C & H Detection (Liebig): — C $\rightarrow$ CO$_2$ (limewater milky), H $\rightarrow$ H$_2$O (anhydrous CuSO$_4$ white $\rightarrow$ blue).\n- N, S, Halogens (Lassaigne): Fuse with Na $\rightarrow$ SFE (NaCN, Na$_2$S, NaX).\n - N: SFE + FeSO$_4$ + FeCl$_3$ $\rightarrow$ Prussian Blue (Fe$_4$[Fe(CN)$_6$]$_3$).\n - N + S: SFE + FeCl$_3$ $\rightarrow$ Blood Red (Fe(SCN)$_3$).\n - S: SFE + Pb(OAc)$_2$ $\rightarrow$ Black PbS; or SFE + Na$_2$[Fe(CN)$_5$NO] $\rightarrow$ Violet.\n - Halogens: SFE + dil. HNO$_3$ + AgNO$_3$ $\rightarrow$ AgCl (white, sol. in NH$_4$OH), AgBr (pale yellow, sparingly sol.), AgI (yellow, insol.).\n- P Detection: Oxidize to PO$_4^{3-}$, then + (NH$_4$)$_2$MoO$_4$ + HNO$_3$ $\rightarrow$ Yellow ppt. ((NH$_4$)$_3$PO$_4 \cdot 12$MoO$_3$).\n- Quantitative Formulas:\n - $\%C = \frac{12}{44} \times \frac{\text{mass of CO}_2}{\text{mass of org. comp.}} \times 100$\n - $\%H = \frac{2}{18} \times \frac{\text{mass of H}_2\text{O}}{\text{mass of org. comp.}} \times 100$\n - $\%N_{\text{Dumas}} = \frac{28}{22400} \times \frac{\text{Vol. of N}_2\text{ at STP}}{\text{mass of org. comp.}} \times 100$\n - $\%N_{\text{Kjeldahl}} = \frac{1.4 \times M_{\text{acid}} \times V_{\text{acid reacted}}}{\text{mass of org. comp.}}$ (simplified)\n - $\%X = \frac{\text{Atomic mass of X}}{\text{Molar mass of AgX}} \times \frac{\text{mass of AgX}}{\text{mass of org. comp.}} \times 100$\n - $\%S = \frac{32}{233} \times \frac{\text{mass of BaSO}_4}{\text{mass of org. comp.}} \times 100$\n- Kjeldahl Limitations: Not for nitro, azo, pyridine N.

2-Minute Revision

Qualitative and quantitative analysis helps us identify and measure elements in organic compounds. For qualitative detection of carbon and hydrogen, we burn the compound with copper oxide; CO$_2$ turns limewater milky, and H$_2$O turns anhydrous copper sulfate blue.

For nitrogen, sulfur, and halogens, Lassaigne's test is key: fuse the compound with sodium to form a water-soluble extract (SFE). Nitrogen gives a Prussian blue color with FeSO$_4$/FeCl$_3$. If sulfur is also present, a blood-red color with FeCl$_3$ indicates thiocyanate.

Sulfur alone gives a black precipitate with lead acetate or a violet color with sodium nitroprusside. Halogens (Cl, Br, I) form silver halides (AgCl, AgBr, AgI) with AgNO$_3$ after acidifying the SFE with HNO$_3$, distinguishable by color and solubility in ammonia.

Phosphorus is detected by oxidizing it to phosphate, which then forms a yellow precipitate with ammonium molybdate. Quantitative analysis involves precise measurements. Carbon and hydrogen are estimated by Liebig's method, weighing the CO$_2$ and H$_2$O produced from combustion.

Nitrogen is estimated by the Dumas method (measuring N$_2$ gas volume at STP) or Kjeldahl's method (titrating liberated ammonia), though Kjeldahl's is not suitable for nitro, azo, or pyridine compounds.

Halogens and sulfur are estimated by the Carius method, where they are converted to silver halides (AgX) and barium sulfate (BaSO$_4$) respectively, and then weighed. Oxygen is typically found by difference.

Mastering these tests, their observations, and calculation formulas is vital for NEET.

5-Minute Revision

Let's quickly review the essential aspects of qualitative and quantitative analysis for NEET. \n\nQualitative Analysis (What's there?):\n1. Carbon & Hydrogen: Heat organic compound with CuO. C $\rightarrow$ CO$_2$ (turns limewater milky, Ca(OH)$_2$ + CO$_2$ $\rightarrow$ CaCO$_3$).

H $\rightarrow$ H$_2$O (turns anhydrous CuSO$_4$ white $\rightarrow$ blue). This is a fundamental test.\n2. Nitrogen, Sulfur, Halogens (Lassaigne's Test): This is crucial. Fuse the organic compound with sodium metal to convert covalent N, S, X into ionic NaCN, Na$_2$S, NaX.

Extract with water to get Sodium Fusion Extract (SFE).\n * Nitrogen: SFE + FeSO$_4$ (freshly prepared) + FeCl$_3$ $\rightarrow$ Prussian Blue (Fe$_4$[Fe(CN)$_6$]$_3$).\n * Nitrogen + Sulfur: If both are present, NaSCN is formed.

SFE + FeCl$_3$ $\rightarrow$ Blood Red (Fe(SCN)$_3$). This is a key distinction.\n * Sulfur: SFE + Pb(CH$_3$COO)$_2$ (lead acetate) $\rightarrow$ Black PbS. Or SFE + Na$_2$[Fe(CN)$_5$NO] (sodium nitroprusside) $\rightarrow$ Violet color.

\n * Halogens (Cl, Br, I): Acidify SFE with dilute HNO$_3$ (to remove NaCN/Na$_2$S interference) + AgNO$_3$ $\rightarrow$ AgCl (white ppt, soluble in NH$_4$OH), AgBr (pale yellow ppt, sparingly soluble), AgI (yellow ppt, insoluble).

Remember the solubility differences.\n3. Phosphorus: Oxidize organic P to phosphate (PO$_4^{3-}$). Then add ammonium molybdate + conc. HNO$_3$ $\rightarrow$ Yellow precipitate of ammonium phosphomolybdate ((NH$_4$)$_3$PO$_4 \cdot 12$MoO$_3$).

\n\nQuantitative Analysis (How much is there?):\n1. Carbon & Hydrogen (Liebig's Method): Burn known mass ($w$) of compound in O$_2$. Weigh H$_2$O ($w_1$) absorbed by anhydrous CaCl$_2$ and CO$_2$ ($w_2$) absorbed by KOH.

\n * $\%C = \frac{12}{44} \times \frac{w_2}{w} \times 100$\n * $\%H = \frac{2}{18} \times \frac{w_1}{w} \times 100$\n2. Nitrogen:\n * Dumas Method: Convert N to N$_2$ gas. Measure volume ($V$) at given T, P.

Convert to $V'$ at STP. \n * $\%N = \frac{28}{22400} \times \frac{V'}{w} \times 100$. (Universal for all N compounds).\n * Kjeldahl's Method: Convert N to (NH$_4$)$_2$SO$_4$, then to NH$_3$. Absorb NH$_3$ in standard acid, back-titrate.

\n * $\%N = \frac{1.4 \times M_{\text{acid}} \times V_{\text{acid reacted with NH}_3}}{w}$. (Not for nitro, azo, pyridine N).\n3. Halogens (Carius Method): Heat compound ($w$) with fuming HNO$_3$ + AgNO$_3$.

Weigh AgX ($w_1$). \n * $\%X = \frac{\text{Atomic mass of X}}{\text{Molar mass of AgX}} \times \frac{w_1}{w} \times 100$\n4. Sulfur (Carius Method): Heat compound ($w$) with fuming HNO$_3$. Add BaCl$_2$.

Weigh BaSO$_4$ ($w_1$). \n * $\%S = \frac{32}{233} \times \frac{w_1}{w} \times 100$\n5. Phosphorus (Carius Method): Oxidize P to H$_3$PO$_4$. Precipitate as MgNH$_4$PO$_4$, ignite to Mg$_2$P$_2$O$_7$ ($w_1$).

\n * $\%P = \frac{2 \times 31}{222} \times \frac{w_1}{w} \times 100$\n6. Oxygen: By difference: $\%O = 100 - (\%C + \%H + \%N + \%S + \%X + \%P)$.\n\nFocus on understanding the principles, remembering the key reagents/observations, and practicing the calculation formulas.

Pay attention to the limitations of each method, especially Kjeldahl's.

Prelims Revision Notes

Qualitative Analysis - Element Detection\n* Carbon & Hydrogen:\n * Method: Liebig's combustion. Organic compound + CuO $\xrightarrow{\text{heat}}$ CO$_2$ + H$_2$O.\n * Detection of CO$_2$ (Carbon): Passes through limewater (Ca(OH)$_2$). Turns milky due to CaCO$_3$ precipitate. Reaction: Ca(OH)$_2$ + CO$_2$ $\rightarrow$ CaCO$_3 \downarrow$ + H$_2$O.\n * Detection of H$_2$O (Hydrogen): Passes through anhydrous copper sulfate (white). Turns blue due to CuSO$_4 \cdot 5$H$_2$O. Reaction: CuSO$_4$ + 5H$_2$O $\rightarrow$ CuSO$_4 \cdot 5$H$_2$O.\n* Nitrogen, Sulfur, Halogens (Lassaigne's Test):\n * Principle: Convert covalent elements to ionic salts by fusion with Na metal. Na + C + N $\rightarrow$ NaCN; 2Na + S $\rightarrow$ Na$_2$S; Na + X $\rightarrow$ NaX.\n * Sodium Fusion Extract (SFE): Boil fused mass with distilled water, filter.\n * Detection of Nitrogen: SFE + FeSO$_4$ (fresh) + FeCl$_3$ $\rightarrow$ Prussian Blue (Fe$_4$[Fe(CN)$_6$]$_3$).\n * Detection of Sulfur: SFE + Pb(CH$_3$COO)$_2$ $\rightarrow$ Black PbS. OR SFE + Na$_2$[Fe(CN)$_5$NO] $\rightarrow$ Violet color.\n * Detection of N & S (together): SFE + FeCl$_3$ $\rightarrow$ Blood Red Fe(SCN)$_3$ (due to NaSCN formation).\n * Detection of Halogens: Acidify SFE with dilute HNO$_3$ (to remove NaCN/Na$_2$S) + AgNO$_3$.\n * Cl: White ppt. (AgCl), soluble in NH$_4$OH.\n * Br: Pale yellow ppt. (AgBr), sparingly soluble in NH$_4$OH.\n * I: Yellow ppt. (AgI), insoluble in NH$_4$OH.\n* Phosphorus Detection:\n * Method: Oxidize P to phosphate (PO$_4^{3-}$). Then add (NH$_4$)$_2$MoO$_4$ + conc. HNO$_3$.\n * Observation: Yellow precipitate of ammonium phosphomolybdate ((NH$_4$)$_3$PO$_4 \cdot 12$MoO$_3$).\n\n### Quantitative Analysis - Element Estimation\n* Carbon & Hydrogen (Liebig's Method):\n * $\%C = \frac{12}{44} \times \frac{\text{mass of CO}_2}{\text{mass of organic compound}} \times 100$\n * $\%H = \frac{2}{18} \times \frac{\text{mass of H}_2\text{O}}{\text{mass of organic compound}} \times 100$\n* Nitrogen:\n * Dumas Method: Universal. N $\rightarrow$ N$_2$ gas. Measure volume ($V$) at given T, P. Convert to $V'$ at STP.\n * $\%N = \frac{28}{22400} \times \frac{V'\text{ (mL at STP)}}{\text{mass of organic compound (g)}} \times 100$\n * Kjeldahl's Method: For amines, amides. N $\rightarrow$ (NH$_4$)$_2$SO$_4$ $\rightarrow$ NH$_3$. Titrate NH$_3$ with standard acid.\n * Limitations: Not for nitro, azo, or pyridine N.\n * $\%N = \frac{1.4 \times M_{\text{acid}} \times V_{\text{acid reacted with NH}_3}}{\text{mass of organic compound}}$\n* Halogens (Carius Method):\n * X $\rightarrow$ AgX. Weigh AgX.\n * $\%X = \frac{\text{Atomic mass of X}}{\text{Molar mass of AgX}} \times \frac{\text{mass of AgX}}{\text{mass of organic compound}} \times 100$\n* Sulfur (Carius Method):\n * S $\rightarrow$ BaSO$_4$. Weigh BaSO$_4$.\n * $\%S = \frac{32}{233} \times \frac{\text{mass of BaSO}_4}{\text{mass of organic compound}} \times 100$\n* Phosphorus (Carius Method):\n * P $\rightarrow$ Mg$_2$P$_2$O$_7$. Weigh Mg$_2$P$_2$O$_7$.\n * $\%P = \frac{2 \times 31}{222} \times \frac{\text{mass of Mg}_2\text{P}_2\text{O}_7}{\text{mass of organic compound}} \times 100$\n* Oxygen: By difference: $\%O = 100 - (\%C + \%H + \%N + \%S + \%X + \%P)$.

Vyyuha Quick Recall

Lassaigne's NSH: Nice Salty Halogens.\nNitrogen: Prussian Blue (for Nice). \nSulfur: Black PbS or Violet Nitroprusside (for Salty). \nHalogens: White, Pale Yellow, Yellow AgX (for Halogens).