Find the best tutors and institutes for Class 11 Tuition
Q8.31:
Why is it necessary to use acetic acid and not sulphuric acid for acidification of sodium extract for testing sulphur by lead acetate test?
Solution :
Initial Theoretical Framework
The detection of sulfur in an organic compound via the sodium fusion extract (Lassaigne's test) involves the conversion of organic sulfur into sodium sulfide ($Na_2S$). To test for the presence of sulfide ions ($S^{2-}$), lead acetate solution is added to the sodium extract.
The chemical reaction for the detection of sulfur is:
$Na_2S + (CH_3COO)_2Pb \rightarrow PbS_{(s)} + 2CH_3COONa$
This reaction forms a black precipitate of lead sulfide ($PbS$), which confirms the presence of sulfur [Per the principles of qualitative inorganic analysis].
Step 1: Role of Acidification in the Test
Sodium fusion extract is typically alkaline due to the presence of unreacted sodium metal or excess sodium hydroxide produced during the fusion process. Acidification is required to ensure that any potential interfering species are neutralized and that the solution is slightly acidic to facilitate the precipitation reaction without causing secondary side reactions [By the Law of Mass Action and solubility product principles].
Step 2: Analysis of Sulfuric Acid ($H_2SO_4$) Usage
If sulfuric acid ($H_2SO_4$) were used for acidification, the following chemical interference would occur:
$H_2SO_4 + (CH_3COO)_2Pb \rightarrow PbSO_{4(s)} + 2CH_3COOH$
Consequences of using $H_2SO_4$:
- Precipitation interference: Lead(II) sulfate ($PbSO_4$) is a white, insoluble precipitate. Its formation would obscure the observation of the black lead sulfide ($PbS$) precipitate, leading to a false negative or ambiguous result.
- Sulfate contamination: If the organic compound contained sulfate groups, adding $H_2SO_4$ would further complicate the analysis by shifting the solubility equilibria.
Step 3: Analysis of Acetic Acid ($CH_3COOH$) Usage
Acetic acid is a weak acid. Its utilization provides two distinct advantages:
- Compatibility: Lead(II) acetate is the reagent of choice because lead(II) ethanoate is soluble in water. Adding acetic acid suppresses the ionization of the acetate ions, maintaining a stable medium.
- Lack of Interfering Anions: Unlike sulfate ions ($SO_4^{2-}$), acetate ions ($CH_3COO^-$) do not form an insoluble precipitate with lead ions ($Pb^{2+}$). Thus, the black precipitate of $PbS$ is formed without any competing white precipitate contamination [Per the Common Ion Effect and Selective Precipitation criteria].
Comparison Table of Acidifying Agents
| Acidifying Agent | Product with Lead Reagent | Resulting Observation |
|---|---|---|
| Sulfuric Acid ($H_2SO_4$) | $PbSO_4$ (White ppt) | Interferes with black $PbS$ visibility |
| Acetic Acid ($CH_3COOH$) | $Pb(CH_3COO)_2$ (Soluble) | Clear observation of black $PbS$ |
Geometric Representation of the Precipitation Conflict
Final Solution: Acetic acid is used because it does not form an insoluble precipitate with lead ions, whereas sulfuric acid would form white lead(II) sulfate ($PbSO_4$), which interferes with the identification of the black lead(II) sulfide ($PbS$) precipitate.
More Questions from Class 11 Chemistry Organic Chemistry – Some Basic Principles and Techniques EXERCISES
- Q8.1: What are hybridisation states of each carbon atom in the following compounds ? $CH_2=C=O$, $CH_3CH=CH_2$, $(CH_3)_2CO$, $CH_2=CHCN$, $C_6H_6$
- Q8.10: Explain why alkyl groups act as electron donors when attached to a $\pi$ system.
- Q8.11 (a): Draw the resonance structures for the following compounds. Show the electron shift using curved-arrow notation. (a) $C_6H_5OH$
- Q8.11 (b): Draw the resonance structures for the following compounds. Show the electron shift using curved-arrow notation. (b) $C_6H_5NO_2$
- Q8.11 (c): Draw the resonance structures for the following compounds. Show the electron shift using curved-arrow notation. (c) $CH_3CH=CHCHO$
- Q8.11 (d): Draw the resonance structures for the following compounds. Show the electron shift using curved-arrow notation. (d) $C_6H_5–CHO$
- Q8.11 (e): Draw the resonance structures for the following compounds. Show the electron shift using curved-arrow notation. (e) $C_6H_5–C^+H_2$
- Q8.11 (f): Draw the resonance structures for the following compounds. Show the electron shift using curved-arrow notation. (f) $CH_3CH=CHC^+H_2$
- Q8.12: What are electrophiles and nucleophiles ? Explain with examples.
- Q8.13 (a): Identify the reagents shown in bold in the following equations as nucleophiles or electrophiles: (a) $CH_3COOH + HO^– \rightarrow CH_3COO^–+H_2O$
- Q8.13 (b): Identify the reagents shown in bold in the following equations as nucleophiles or electrophiles: (b) $CH_3COCH_3+ C^–N \rightarrow (CH_3)_2C(CN)(OH)$
- Q8.13 (c): Identify the reagents shown in bold in the following equations as nucleophiles or electrophiles: (c) $C_6H_6 + CH_3C^+O \rightarrow C_6H_5COCH_3$
- Q8.14 (a): Classify the following reactions in one of the reaction type studied in this unit. (a) $CH_3CH_2Br + HS^– \rightarrow CH_3CH_2SH + Br^–$
- Q8.14 (b): Classify the following reactions in one of the reaction type studied in this unit. (b) $(CH_3)_2C = CH_2 + HCl \rightarrow (CH_3)_2ClC – CH_3$
- Q8.14 (c): Classify the following reactions in one of the reaction type studied in this unit. (c) $CH_3CH_2Br + HO^– \rightarrow CH_2 = CH_2 + H_2O + Br^–$
- Q8.14 (d): Classify the following reactions in one of the reaction type studied in this unit. (d) $(CH_3)_3C– CH_2OH + HBr \rightarrow (CH_3)_2CBrCH_2CH_2CH_3 + H_2O$
- Q8.15: What is the relationship between the members of following pairs of structures ? Are they structural or geometrical isomers or resonance contributors ? (c)
- Q8.15: What is the relationship between the members of following pairs of structures ? Are they structural or geometrical isomers or resonance contributors ? (b)
- Q8.15: What is the relationship between the members of following pairs of structures ? Are they structural or geometrical isomers or resonance contributors ? (a)
- Q8.16: For the following bond cleavages, use curved-arrows to show the electron flow and classify each as homolysis or heterolysis. Identify reactive intermediate produced as free radical, carbocation and carbanion. (c)
- Q8.16: For the following bond cleavages, use curved-arrows to show the electron flow and classify each as homolysis or heterolysis. Identify reactive intermediate produced as free radical, carbocation and carbanion. (d)
- Q8.16: For the following bond cleavages, use curved-arrows to show the electron flow and classify each as homolysis or heterolysis. Identify reactive intermediate produced as free radical, carbocation and carbanion. (b)
- Q8.16: For the following bond cleavages, use curved-arrows to show the electron flow and classify each as homolysis or heterolysis. Identify reactive intermediate produced as free radical, carbocation and carbanion. (a)
- Q8.17 (a): Explain the terms Inductive and Electromeric effects. Which electron displacement effect explains the following correct orders of acidity of the carboxylic acids? (a) $Cl_3CCOOH > Cl_2CHCOOH > ClCH_2COOH$
- Q8.17 (b): Explain the terms Inductive and Electromeric effects. Which electron displacement effect explains the following correct orders of acidity of the carboxylic acids? (b) $CH_3CH_2COOH > (CH_3)_2CHCOOH > (CH_3)_3C.COOH$
- Q8.18 (a): Give a brief description of the principles of the following techniques taking an example in each case. (a) Crystallisation
- Q8.18 (b): Give a brief description of the principles of the following techniques taking an example in each case. (b) Distillation
- Q8.18 (c): Give a brief description of the principles of the following techniques taking an example in each case. (c) Chromatography
- Q8.19: Describe the method, which can be used to separate two compounds with different solubilities in a solvent S.
- Q8.2: Indicate the $\sigma$ and $\pi$ bonds in the following molecules : $C_6H_6$, $C_6H_{12}$, $CH_2Cl_2$, $CH_2=C=CH_2$, $CH_3NO_2$, $HCONHCH_3$
- Q8.20: What is the difference between distillation, distillation under reduced pressure and steam distillation ?
- Q8.21: Discuss the chemistry of Lassaigne’s test.
- Q8.22: Differentiate between the principle of estimation of nitrogen in an organic compound by (i) Dumas method and (ii) Kjeldahl’s method.
- Q8.23: Discuss the principle of estimation of halogens, sulphur and phosphorus present in an organic compound.
- Q8.24: Explain the principle of paper chromatography.
- Q8.25: Why is nitric acid added to sodium extract before adding silver nitrate for testing halogens?
- Q8.26: Explain the reason for the fusion of an organic compound with metallic sodium for testing nitrogen, sulphur and halogens.
- Q8.27: Name a suitable technique of separation of the components from a mixture of calcium sulphate and camphor.
- Q8.28: Explain, why an organic liquid vaporises at a temperature below its boiling point in its steam distillation ?
- Q8.29: Will $CCl_4$ give white precipitate of AgCl on heating it with silver nitrate? Give reason for your answer.
- Q8.3: Write bond line formulas for : Isopropyl alcohol, 2,3-Dimethylbutanal, Heptan-4-one.
- Q8.30: Why is a solution of potassium hydroxide used to absorb carbon dioxide evolved during the estimation of carbon present in an organic compound?
- Q8.32: An organic compound contains 69% carbon and 4.8% hydrogen, the remainder being oxygen. Calculate the masses of carbon dioxide and water produced when 0.20 g of this substance is subjected to complete combustion.
- Q8.33: A sample of 0.50 g of an organic compound was treated according to Kjeldahl’s method. The ammonia evolved was absorbed in 50 ml of 0.5 M $H_2SO_4$. The residual acid required 60 mL of 0.5 M solution of NaOH for neutralisation. Find the percentage composition of nitrogen in the compound.
- Q8.34: 0.3780 g of an organic chloro compound gave 0.5740 g of silver chloride in Carius estimation. Calculate the percentage of chlorine present in the compound.
- Q8.35: In the estimation of sulphur by Carius method, 0.468 g of an organic sulphur compound afforded 0.668 g of barium sulphate. Find out the percentage of sulphur in the given compound.
- Q8.36: In the organic compound $CH_2 = CH – CH_2 – CH_2 – C \equiv CH$, the pair of hydridised orbitals involved in the formation of: $C_2 – C_3$ bond is:
- Q8.37: In the Lassaigne’s test for nitrogen in an organic compound, the Prussian blue colour is obtained due to the formation of:
- Q8.38: Which of the following carbocation is most stable ?
- Q8.39: The best and latest technique for isolation, purification and separation of organic compounds is:
- Q8.4: Give the IUPAC names of the following compounds : (a)
- Q8.4: Give the IUPAC names of the following compounds : (b)
- Q8.4: Give the IUPAC names of the following compounds : (c)
- Q8.4: Give the IUPAC names of the following compounds : (d)
- Q8.4: Give the IUPAC names of the following compounds : (e)
- Q8.4 (f): Give the IUPAC names of the following compounds : (f) $Cl_2CHCH_2OH$
- Q8.40: The reaction: $CH_3CH_2I + KOH(aq) \rightarrow CH_3CH_2OH + KI$ is classified as :
- Q8.5 (a): Which of the following represents the correct IUPAC name for the compounds concerned ? (a) 2,2-Dimethylpentane or 2-Dimethylpentane
- Q8.5 (b): Which of the following represents the correct IUPAC name for the compounds concerned ? (b) 2,4,7-Trimethyloctane or 2,5,7-Trimethyloctane
- Q8.5 (c): Which of the following represents the correct IUPAC name for the compounds concerned ? (c) 2-Chloro-4-methylpentane or 4-Chloro-2-methylpentane
- Q8.5 (d): Which of the following represents the correct IUPAC name for the compounds concerned ? (d) But-3-yn-1-ol or But-4-ol-1-yne.
- Q8.6 (a): Draw formulas for the first five members of each homologous series beginning with the following compounds. (a) H–COOH
- Q8.6 (b): Draw formulas for the first five members of each homologous series beginning with the following compounds. (b) $CH_3COCH_3$
- Q8.6 (c): Draw formulas for the first five members of each homologous series beginning with the following compounds. (c) H–CH=CH2
- Q8.7 (a): Give condensed and bond line structural formulas and identify the functional group(s) present, if any, for : (a) 2,2,4-Trimethylpentane
- Q8.7 (b): Give condensed and bond line structural formulas and identify the functional group(s) present, if any, for : (b) 2-Hydroxy-1,2,3-propanetricarboxylic acid
- Q8.7 (c): Give condensed and bond line structural formulas and identify the functional group(s) present, if any, for : (c) Hexanedial
- Q8.8: Identify the functional groups in the following compounds (c)
- Q8.8: Identify the functional groups in the following compounds (b)
- Q8.8: Identify the functional groups in the following compounds (a)
- Q8.9: Which of the two: $O_2NCH_2CH_2O^–$ or $CH_3CH_2O^–$ is expected to be more stable and why?
CBSE Solutions for Class 11 Chemistry Organic Chemistry – Some Basic Principles and Techniques
Chapters in CBSE - Class 11 Chemistry
Top Tutors who teach Organic Chemistry – Some Basic Principles and Techniques
Top Tutor
Sector 33, Gurgaon
18 yrs of
Exp
12 students
796 hrs of classes completed
1000
per hour
Online Classes
Tutor's home
Expert Chemistry Mentor for IIT-JEE | NEET | CBSE – 18+ Years of Experience Hi, I’m Anuranjita. I am a Senior Chemistry Expert with over 18 years of experience at India's leading institutes like Aakash, FIITJEE, and TIME and hold an M.Tech. degree from Anna University, Chennai—one of the top 10 engineering universities in India. I have refined a highly structured, result-oriented teaching methodology designed to crack • IIT-JEE (Main & Advanced) • NEET • Score 100% in CBSE Board exams. (Class 11 & 12 – Chemistry) Over the years, I have trained thousands of students, many of whom have successfully secured admissions into AIIMS, IITs, NITs, and other top medical and engineering colleges across India. Here is what makes my classes uniquely effective: • Interactive and concept-driven teaching • Focus on building strong fundamentals • Personalized learning based on each student’s pace • Regular doubt-clearing sessions & Chapter-wise tests • Exam-oriented tips and problem-solving strategies • The "Exclusive 5" Batch Model (Personalized Attention) Unlike massive online batches with thousands of students where individual voices get lost, my classes are strictly limited to a maximum of 5 students. This ensures that every single student receives 1-on-1 level attention, and I can monitor each student's real-time progress, strengths, and weaknesses. • Rigorous Testing & PYQ Practice: Chapter-wise tests modeled strictly on the latest NTA patterns, alongside exhaustive solving of past years' questions (PYQs). Looking forward to helping you master chemistry! Best Regards Anuranjita
"Chemistry Mam has been very helpful for my daughter’s study. After struggling with the overload at standard coaching institutes, finding a teacher who simplifies complex topics made all the difference. Her passion and willingness to take unplanned classes show how much she truly cares about her students' success. Highly recommended!"
Garia, Kolkata
4 yrs of
Exp
1 student
20 hrs of classes completed
500
per hour
Online Classes
Student's Home
Tutor's home
I can teach and make students understand concepts with fun. So they doesn't feel bored in the class
Top Tutor
Bhagalpur, Bhagalpur
15 yrs of
Exp
4 students
152 hrs of classes completed
600
per hour
Online Classes
A Group of Extra-ordinary students who are ready to fire in the NEET
A good education can change anyone a good teacher can change everything a good teacher is like a candle it consumes itself to light the way for other s the best teacher s are those who show you where to look but don't tell you what to see teaching is one of the profession that I am very thankful to you.
Top Tutor
Manimajra, Panchkula
8 yrs of
Exp
2 students
20 hrs of classes completed
500
per hour
Online Classes
Student's Home
Tutor's home
I am a commerce teacher. Giving home tuitions since 2014. I have degree in M. Com and B. Ed and also certified in CTET ..
Top Tutor
Chembur West, Mumbai
17 yrs of
Exp
9 students
273 hrs of classes completed
750
per hour
Online Classes
Tutor's home
I am a teacher. I provide online and tutor home tuitions. I am M.Sc.(organic chemistry) B.Ed. I have an experience of teaching chemistry in Engineering college , schools (Aditya Birla Public School, Sagar public School),in coaching classes., home tuitions and online tuitions. I have around 15 years teaching experience.
My daughter took demo class on physics from Sheeba ma'am. She explained the topic in detail and helped in resolving the doubts. Overall a helpful session.
Other Subjects in CBSE - Class 11
Download free CBSE - Class 11 Chemistry Organic Chemistry – Some Basic Principles and Techniques EXERCISES worksheets
Download Now
