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CBSE Class 12 Chemistry Important Questions

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Last updated: 21 Sep 2026 GKSchools Editorial CBSE and state board classes

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Board Exam 2026 - High Priority MCQs with Detailed Answers

🎯 Most Important 📚 NCERT Based ✅ Board Pattern

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⏱️ Questions

⭐ HIGH PRIORITY

The unit of rate constant for a zero order reaction is:

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mol L⁻¹ s⁻¹ s⁻¹ L mol⁻¹ s⁻¹ mol² L⁻² s⁻¹

✅ Correct Answer: (a) mol L⁻¹ s⁻¹

For zero order reaction: Rate = k[A]⁰ = k

Explanation: Units of k = Units of rate = mol L⁻¹ s⁻¹
Remember: Zero order → mol L⁻¹ s⁻¹, First order → s⁻¹, Second order → L mol⁻¹ s⁻¹

⭐ HIGH PRIORITY

Which of the following is a primary halide?

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Isopropyl bromide tert-butyl chloride n-propyl chloride sec-butyl bromide

✅ Correct Answer: (c) n-propyl chloride

CH₃-CH₂-CH₂-Cl (1° halide)

Explanation: In primary halide, the carbon bearing halogen is attached to only one carbon atom. n-propyl chloride has the -Cl on a primary carbon.

⭐ HIGH PRIORITY

The coordination number of Ni in [Ni(NH₃)₆]²⁺ is:

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2 4 6 8

✅ Correct Answer: (c) 6

Six NH₃ ligands are attached to central Ni²⁺ ion

Explanation: Coordination number = Number of ligands directly attached to the central metal ion. Here, 6 ammonia molecules are coordinated with Ni²⁺.

⭐ HIGH PRIORITY

Aldehydes can be distinguished from ketones using:

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Fehling's test Benedict's test Tollen's test All of the above

✅ Correct Answer: (d) All of the above

Aldehydes give positive test, ketones don't

Explanation: All three tests distinguish aldehydes from ketones. Aldehydes reduce Fehling's, Benedict's, and Tollen's reagent due to presence of -CHO group. Ketones don't react.

⭐ HIGH PRIORITY

Faraday's constant (F) is equal to:

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96500 C mol⁻¹ 1.6 × 10⁻¹⁹ C 6.022 × 10²³ 8.314 J mol⁻¹ K⁻¹

✅ Correct Answer: (a) 96500 C mol⁻¹

F = e × Nₐ = 1.6 × 10⁻¹⁹ × 6.022 × 10²³ ≈ 96500 C mol⁻¹

Explanation: Faraday constant is the charge carried by one mole of electrons. It's fundamental in electrochemistry calculations.

⭐ HIGH PRIORITY

The half-life of a first order reaction is 30 minutes. What is the rate constant?

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0.0231 min⁻¹ 0.693 min⁻¹ 20.79 min⁻¹ 0.0693 min⁻¹

✅ Correct Answer: (a) 0.0231 min⁻¹

k = 0.693/t₁/₂ = 0.693/30 = 0.0231 min⁻¹

Explanation: For first order reactions: k = 0.693/t₁/₂. This is one of the most important formulas for board exams!

⭐ HIGH PRIORITY

Which of the following has the highest conductivity in aqueous solution?

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0.1 M CH₃COOH 0.1 M NH₄OH 0.1 M NaCl 0.1 M C₆H₁₂O₆

✅ Correct Answer: (c) 0.1 M NaCl

NaCl is a strong electrolyte - completely ionizes

Explanation: NaCl (strong electrolyte) > CH₃COOH, NH₄OH (weak electrolytes) > C₆H₁₂O₆ (non-electrolyte). Higher ionization = higher conductivity.

⭐ HIGH PRIORITY

Identify the chiral molecule:

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2-Bromobutane 2,2-Dibromopropane 1-Bromobutane 2-Bromopropane

✅ Correct Answer: (a) 2-Bromobutane

Contains an asymmetric carbon (chiral center)

Explanation: In 2-bromobutane, C-2 is attached to 4 different groups (H, Br, CH₃, C₂H₅), making it chiral. Chiral molecules show optical isomerism.

⭐ HIGH PRIORITY

The IUPAC name of CH₃-CO-CH₃ is:

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Propanone