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NEET UG

NEET UG MCQ questions — Physics, Chemistry, Botany & Zoology for medical entrance.

904 Q 4 Subjects 12th (PCB)
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Difficulty: All Easy Medium Hard 81–90 of 904
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Q.81 Medium NEET Chemistry Physical Chemistry
Which of the following will shift the equilibrium of the reaction CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g), ΔH = -41 kJ/mol to the right?
A Decreasing temperature
B Increasing pressure
C Adding more CO
D Adding catalyst
Correct Answer:  A. Decreasing temperature
EXPLANATION

This is an exothermic reaction (ΔH negative). By Le Chatelier's principle, decreasing temperature shifts equilibrium towards the products (right), favoring the exothermic forward reaction.

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Q.82 Medium NEET Chemistry Physical Chemistry
For a reversible reaction at equilibrium, if Kc = 4 at 25°C, what is the relationship between forward and reverse rate constants?
A kf = 4kr
B kf = kr
C kf > kr
D kf < kr
Correct Answer:  A. kf = 4kr
EXPLANATION

At equilibrium, Kc = kf/kr. Given Kc = 4, this means kf = 4kr, indicating the forward reaction is faster than the reverse reaction.

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Q.83 Medium NEET Chemistry Physical Chemistry
A buffer solution is prepared by mixing 100 mL of 0.1 M acetic acid (CH₃COOH) and 100 mL of 0.1 M sodium acetate (CH₃COONa). Using Henderson-Hasselbalch equation, if Ka = 1.8 × 10⁻⁵, what is the pH?
A 3.74
B 4.74
C 5.74
D 6.74
Correct Answer:  B. 4.74
EXPLANATION

pH = pKa + log([A⁻]/[HA]). pKa = -log(1.8 × 10⁻⁵) ≈ 4.74. Since [A⁻] = [HA] (equal concentrations), log(1) = 0. Therefore, pH = 4.74.

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Q.84 Easy NEET Chemistry Physical Chemistry
What is the relationship between ΔG, ΔH, and ΔS?
A ΔG = ΔH - TΔS
B ΔG = ΔH + TΔS
C ΔG = TΔS - ΔH
D ΔG = ΔH × TΔS
Correct Answer:  A. ΔG = ΔH - TΔS
EXPLANATION

The Gibbs free energy equation is ΔG = ΔH - TΔS, where T is temperature in Kelvin. This is fundamental to predicting spontaneity.

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Q.85 Easy NEET Chemistry Physical Chemistry
For the reaction: 2A + B → C, if the rate law is Rate = k[A]²[B], what is the overall order of the reaction?
A 1
B 2
C 3
D 4
Correct Answer:  C. 3
EXPLANATION

The overall order of a reaction is the sum of all exponents in the rate law. Here, order = 2 + 1 = 3.

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Q.86 Easy NEET Chemistry Physical Chemistry
The rate constant of a first-order reaction is 0.693 s⁻¹. What is the half-life of this reaction?
A 0.5 s
B 1 s
C 2 s
D 4 s
Correct Answer:  B. 1 s
EXPLANATION

For a first-order reaction, t₁/₂ = 0.693/k = 0.693/0.693 = 1 s. This uses the relationship ln(2) ≈ 0.693.

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Q.87 Easy NEET Chemistry Physical Chemistry
What is the pH of a 0.01 M HCl solution?
A 1
B 2
C 3
D 4
Correct Answer:  B. 2
EXPLANATION

HCl is a strong acid and completely dissociates. [H⁺] = 0.01 M = 10⁻² M. Therefore, pH = -log[H⁺] = -log(10⁻²) = 2.

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Q.88 Easy NEET Chemistry Physical Chemistry
Which of the following is the correct expression for the equilibrium constant (Kc) for the reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)?
A Kc = [NH₃]²/([N₂][H₂]³)
B Kc = [N₂][H₂]³/[NH₃]²
C Kc = [NH₃]/([N₂][H₂])
D Kc = [N₂] + [H₂]/[NH₃]
Correct Answer:  A. Kc = [NH₃]²/([N₂][H₂]³)
EXPLANATION

The equilibrium constant expression follows the law of mass action where Kc = [products]^coefficients/[reactants]^coefficients. For this reaction, Kc = [NH₃]²/([N₂][H₂]³).

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Q.89 Medium NEET Botany Plant Physiology
Which of the following is a characteristic feature of bundle sheath cells in C4 plants?
A Rich in chloroplasts with well-developed grana
B Absence of starch grains
C Tightly packed with few intercellular spaces
D Directly exposed to stomata
Correct Answer:  A. Rich in chloroplasts with well-developed grana
EXPLANATION

Bundle sheath cells in C4 plants contain large chloroplasts with well-developed grana for the Calvin cycle, unlike mesophyll cells.

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Q.90 Medium NEET Botany Plant Physiology
In C4 plants, the first stable product of CO2 fixation is:
A 3-Phosphoglycerate
B Oxaloacetate
C Malate
D Phosphoenolpyruvate
Correct Answer:  B. Oxaloacetate
EXPLANATION

In the Hatch-Slack pathway, PEP carboxylase catalyzes formation of oxaloacetate (4-carbon compound) in mesophyll cells.

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