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CUET · CHEMISTRY · PYQ PAPER 2025

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The chemical reactions proceed at different rates which depend on various factors like concentration, temperature, etc. Rate law is an expression which relates the rate of reaction with concentration of various reacting species in the reaction. For a first order reaction, the concentration of the reactant after time \(t\) is related with its initial concentration by the relation, \(C=C_0 e^{-k t}\). The rate constants for different orders have different expressions. For a first order reaction, \(k=\frac{2.303}{t} \log \frac{a}{a-x}\), where \(a\) is the initial concentration and \(x\) is the extent of reaction.
A reaction which is first order with respect to the reactant \(A\) has a rate constant of \(6 min^{-1}\). If the reaction is started with \([A]=5.0 mol L ^{-1}\) when would \([A]\) reach the value of \(0.05 mol L ^{-1}\)

  1. A 0.7676 min
  2. B 0.4656 min
  3. C 0.0765 min
  4. D 0.54 min
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Answer & Solution

Correct Answer

(A) 0.7676 min

Step-by-step Solution

Detailed explanation

\(t = \frac{2.303}{k} \log \frac{[A]_0}{[A]}\) \(t = \frac{2.303}{6} \log \frac{5.0}{0.05}\) \(t = \frac{2.303}{6} \log(100)\) \(t = \frac{2.303}{6} \times 2\) \(t = \frac{4.606}{6}\) \(t = 0.7676 \, min\)
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