Over 100 Short Q&A for Redox and Acid-Base Titrations (Class 12 NCERT Lab Manual)

 

Over 100 Short Q&A for Redox and Acid-Base Titrations (Class 12 NCERT Lab Manual)

Volumetric Analysis (Redox and Acid-Base Titrations) is crucial for understanding stoichiometry and solution preparation. 

Part 1: Basic Volumetric Concepts

Q. No.	Question	Answer
1	What is Volumetric Analysis?	A quantitative analysis technique based on measuring the volume of a solution of known concentration required to react completely with a solution of unknown concentration.
2	Define Titration.	The process of adding the solution from the burette to the solution in the conical flask until the reaction is complete.
3	Define Titre.	The volume of solution taken from the burette during the titration.
4	What is the Endpoint of a titration?	The point where the indicator changes color, signaling the completion of the reaction.
5	What is the Equivalence Point of a titration?	The point where the moles (or equivalents) of the reactant from the burette exactly equal the moles (or equivalents) of the reactant in the flask.
6	What is the ideal relationship between the endpoint and the equivalence point?	They should be as close as possible.
7	Define a Standard Solution.	A solution whose concentration is accurately known.
8	Define a Primary Standard.	A substance that can be obtained in high purity, is stable, and has a known chemical composition (used to make standard solutions directly).
9	Give two examples of Primary Standards used in titrations.	Oxalic Acid (Acid-Base) and Anhydrous Na2​CO3​ (Acid-Base).
10	Define a Secondary Standard.	A substance that cannot be obtained in high purity or is unstable, and must be standardized against a primary standard (e.g., NaOH,KMnO4​).

Part 2: Acid-Base Titrations (Acidimetry and Alkalimetry)

Q. No.	Question	Answer
11	What type of reaction is the basis of an Acid-Base titration?	Neutralization Reaction.
12	What is the fundamental relationship used at the equivalence point in acid-base titration?	NA​VA​=NB​VB​ (or MolesA​=MolesB​ for 1:1 reaction).
13	What is the purpose of phenolphthalein indicator?	To indicate the endpoint in titrations involving a strong base (pH range 8.3−10.5).
14	What color change does phenolphthalein show in a basic solution?	Colorless to Pink.
15	What color change does phenolphthalein show in an acidic solution?	Colorless to Colorless.
16	What is the purpose of methyl orange indicator?	To indicate the endpoint in titrations involving a strong acid (pH range 3.1−4.4).
17	What color change does methyl orange show in an acidic solution?	Yellow to Red.
18	Which indicator is suitable for the titration of a Strong Acid vs. Strong Base (HCl vs NaOH)?	Phenolphthalein or Methyl Orange (endpoint pH≈7, both work).
19	Which indicator is suitable for the titration of a Weak Acid vs. Strong Base (Oxalic vs NaOH)?	Phenolphthalein (endpoint pH>7, basic range).
20	Which indicator is suitable for the titration of a Strong Acid vs. Weak Base (HCl vs Na2​CO3​)?	Methyl Orange (endpoint pH<7, acidic range).
21	Why is NaOH considered a secondary standard?	It is hygroscopic (absorbs moisture) and absorbs CO2​ from the air, making its concentration unstable.
22	Why is Oxalic Acid considered a primary standard?	It is stable, non-hygroscopic, and available in high purity.
23	Why is the conical flask rinsed only with water before titration?	To avoid changing the moles of the substance being analyzed (water doesn't add moles of solute).
24	Why is the burette rinsed with the titrant solution before titration?	To ensure the concentration of the liquid dispensed is not diluted by residual water.
25	What is the common source of error when reading the burette?	Parallax error (not reading at eye level).
26	What part of the liquid meniscus should be read in a burette for colorless solutions?	The lower meniscus.

Part 3: Redox Titrations (Permanganometry)

Q. No.	Question	Answer
27	What type of reaction is the basis of a Redox titration?	Oxidation-Reduction (electron transfer).
28	What is the primary oxidizing agent used in Permanganometry experiments?	Potassium Permanganate (KMnO4​).
29	Why is KMnO4​ considered a secondary standard?	It is difficult to obtain in high purity and may contain MnO2​ impurity.
30	Against which primary standard is KMnO4​ usually standardized?	Oxalic Acid or Mohr's Salt (Ferrous Ammonium Sulphate).
31	Why must KMnO4​ titrations be carried out in an acidic medium?	KMnO4​ is the strongest oxidizing agent in acidic solution (MnO4−​→Mn2+).
32	What is the reducing agent when KMnO4​ reacts with Oxalic Acid?	Oxalate ion (C2​O42−​).
33	What are the oxidation products of Oxalic Acid in this titration?	Carbon Dioxide (CO2​) and Water (C2​O42−​→2CO2​).
34	What is the oxidation number change for Mn in acidic medium (KMnO4​)?	From +7 to +2 (n-factor =5).
35	What is the oxidation number change for Carbon in Oxalic Acid?	From +3 to +4 (n-factor per C2​O42−​ unit =2).
36	What is the self-indicator in the KMnO4​ vs. Oxalic Acid titration?	KMnO4​ itself.
37	What is the color change at the endpoint of the KMnO4​ titration?	Colorless to Permanent Pale Pink.
38	Why is the solution heated to 60−70∘C during the KMnO4​ vs Oxalic Acid titration?	To increase the reaction rate, as the reaction is slow at room temperature.
39	What is the catalyst for the KMnO4​ vs Oxalic Acid titration?	Mn2+ ions (Autocatalysis).
40	What is the reducing agent when KMnO4​ reacts with Mohr's Salt?	Ferrous ion (Fe2+).
41	What is the oxidation product of the Fe2+ ion?	Ferric ion (Fe3+) (Fe2+→Fe3+).
42	What is the n-factor for Fe2+ in this reaction?	1 (Change from +2 to +3).
43	Why is FeSO4​ (Ferrous Sulphate) preferred as a secondary standard over FeSO4​ alone?	Mohr's Salt (FAS) is more stable and does not readily oxidize in air.
44	What is the disadvantage of using KMnO4​ in a basic medium?	It reduces to brown/black MnO2​, making the endpoint difficult to detect.

Part 4: Calculation and Equivalence Concepts

Q. No.	Question	Answer
45	Define Normality (N).	Number of gram equivalents of solute per litre of solution.
46	Define Equivalent Mass of an acid.	Molar Mass/Basicity (number of replaceable H+ ions).
47	Define Equivalent Mass of a base.	Molar Mass/Acidity (number of replaceable OH− ions).
48	Define Equivalent Mass of an oxidizing/reducing agent.	Molar Mass/Change in Oxidation Number (n-factor).
49	What is the relationship between Normality and Molarity?	N=M×n-factor.
50	What is the equivalent mass of HCl?	36.5 g/eq (Molar Mass/1).
51	What is the equivalent mass of H2​SO4​ in a neutralization reaction?	49.0 g/eq (Molar Mass/2).
52	What is the equivalent mass of KMnO4​ in an acidic medium?	≈31.6 g/eq (Molar Mass/5).
53	What is the equivalent mass of Mohr’s Salt in a redox titration?	Molar Mass/1 (Only Fe2+ is oxidized).
54	How many grams of NaOH (mol mass 40) are in 1 L of 1 N solution?	40 g (Eq Mass=40).
55	How many grams of H2​SO4​ (mol mass 98) are in 1 L of 1 N solution?	49 g (Eq Mass=49).

Part 5: Indicators and Endpoint Detection

Q. No.	Question	Answer
56	What is an Acid-Base Indicator?	A weak organic acid or base whose undissociated form has a different color than its ionic form.
57	What is the mechanism by which an indicator changes color?	The shift in the indicator's dissociation equilibrium due to the change in pH.
58	What pH range should an indicator's color change cover for accurate results?	The color change should occur within the steep portion of the titration curve.
59	Why is the endpoint of CH3​COOH vs NaOH not sharp?	The pH change around the equivalence point is less steep (buffering action of the salt).
60	Can a common indicator (like phenolphthalein) be used in a redox titration?	No, redox indicators are required, or the titrant must be self-indicating (KMnO4​).
61	Give an example of an external indicator used in a redox titration.	Starch solution (used for I2​ titrations).
62	What color complex does Starch form with I2​?	Deep blue.
63	What is a Concordant Reading?	Titre values that agree closely (usually within ±0.1 mL of each other).
64	What is the major error when the color change is observed only after a delay of 30 s?	Overshooting the endpoint (endpoint should be recorded at the first permanent color change).

Part 6: Practical Precautions and Calculations

Q. No.	Question	Answer
65	What is the correct way to add the titrant near the endpoint?	Drop by drop, with continuous swirling.
66	What is the purpose of placing a white tile/paper under the conical flask?	To clearly visualize the color change at the endpoint.
67	What substance should be avoided near a KMnO4​ titration?	Rubber (as KMnO4​ oxidizes rubber), hence glass stopcocks are preferred.
68	Why must the burette tip be free of air bubbles before starting?	Air bubbles introduce volume errors in the titre reading.
69	What is the final step in standardizing a secondary standard solution?	Calculate the exact molarity/normality using the titration reading and the known concentration of the primary standard.
70	How is the percentage purity of a sample determined using titration data?	%Purity=Mass of pure substance​×100/Mass of impure sample
71	If V1​ L of M1​ M acid reacts with V2​ L of M2​ M base (1:1 mole ratio), what is the relationship at the endpoint?	M1​V1​=M2​V2​.

Part 7: More on Permanganometry and Oxalic Acid

Q. No.	Question	Answer
72	What is the full name of Mohr's Salt?	Ferrous Ammonium Sulphate (FeSO4​⋅(NH4​)2​SO4​⋅6H2​O).
73	What is the chemical formula of Oxalic Acid dihydrate?	H2​C2​O4​⋅2H2​O.
74	What happens if H2​SO4​ is not added to the conical flask in the KMnO4​ titration?	The reaction will take place in a neutral/basic medium, reducing KMnO4​ to MnO2​ (brown ppt), obscuring the endpoint.
75	Why is the initial addition of KMnO4​ to hot oxalic acid slow?	The initial reaction is slow; Mn2+ (autocatalyst) has not yet formed in sufficient concentration.
76	Why must the KMnO4​ solution be stored in dark bottles?	Light decomposes KMnO4​ into MnO2​, affecting its concentration.
77	What precaution is necessary regarding HCl when standardizing KMnO4​?	HCl cannot be used for acidification because KMnO4​ will oxidize Cl− to Cl2​.
78	What is the standard concentration used for KMnO4​ solutions in the lab manual?	Usually M/20 or N/10.

Part 8: Additional Titration Concepts

Q. No.	Question	Answer
79	What is a Double Titration?	Titrating a mixture of two components (e.g., NaOH and Na2​CO3​) using two different indicators (phenolphthalein and methyl orange).
80	In the double titration of NaOH and Na2​CO3​ with HCl, what happens at the phenolphthalein endpoint?	NaOH is completely neutralized, and Na2​CO3​ is converted to NaHCO3​.
81	What happens between the phenolphthalein endpoint and the methyl orange endpoint?	NaHCO3​ is converted to H2​CO3​.
82	What is the purpose of adding 1 g of KI in the titration of Cu2+?	I− is oxidized by Cu2+ to I2​ (Iodometry) which is then titrated.
83	Why should the titration of iodine (I2​) be done as soon as it is prepared?	I2​ is volatile and its concentration will change.
84	Which indicator is used in Iodometric titrations?	Starch solution.
85	Why is starch indicator added only when the I2​ color is faint yellow?	Starch forms a stable complex with high I2​ concentration, making the endpoint inaccurate.
86	What is the relationship between the Equivalent Mass of Na2​S2​O3​ and its Molar Mass?	1:1 (Na2​S2​O3​→Na2​S4​O6​ in the reaction).

Part 9: Error Analysis and Standardisation

Q. No.	Question	Answer
87	What is the difference between a systematic error and a random error?	Systematic errors are constant (e.g., faulty balance); Random errors vary (e.g., reading meniscus).
88	What is the main source of random error in titration?	Endpoint detection and meniscus reading.
89	What is the standard pH for a neutral solution at 25∘C?	7.0.
90	Why is standardizing secondary solutions crucial?	To determine their exact concentration due to their unstable or impure nature.
91	What is the primary use of a pipette in titration?	To accurately measure a fixed volume of the solution into the conical flask.
92	What is the primary use of a volumetric flask?	To accurately prepare a solution of a specific volume and known concentration.
93	Why should the pipette never be blown out to dispense the final drop?	The pipette is calibrated to deliver the volume only when the tip is allowed to drain freely.
94	What is the term for the final, permanent color observed?	Permanent Endpoint.

Part 10: Final Revision Points

Q. No.	Question	Answer
95	What is the n-factor of H3​PO4​ in a titration with NaOH?	3 (Tribasic acid).
96	What is the n-factor of H3​BO3​ (Boric Acid)?	1 (Monobasic acid, acts as a Lewis acid).
97	What is the equivalent mass of Na2​CO3​ in a titration against HCl?	Molar Mass/2 (releases 2 Na+ or requires 2 H+).
98	Why is an acidic medium preferred for KMnO4​ titrations?	The reduction is cleanest and fastest, and the endpoint is sharp (colorless to pink).
99	What is the chemical that causes the pale pink color at the endpoint of a KMnO4​ titration?	Excess MnO4−​ ions (the reducing agent is depleted).
100	Why must the conical flask be swirled continuously during titration?	To ensure complete and homogeneous mixing of the reactants.
101	If the normality is 2 N and the volume is 20 mL, how many equivalents are present?	0.04 equivalents (2 N×0.02 L).
102	What must be done if the initial burette reading is below the 0.0 mL mark?	Record the initial reading as is, but ensure the volume is sufficient for the entire titration.
103	If the pH changes from 4 to 10 near the equivalence point, what acid/base types are involved?	Strong Acid vs. Strong Base (large pH jump).
104	What is the term for the method where the analyte is added to the burette?	Direct Titration.
105	What is the common catalyst added in the standardization of KMnO4​ against Mohr’s Salt?	Dilute H2​SO4​ (provides the acidic medium).