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Chemical Kinetics and Nuclear Chemistry question

2022 · 28 Jun · Shift 1 · Q21
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Chemical Kinetics and Nuclear Chemistry question

2022 · 28 Jun · Shift 1 · Q21

JEE MainChemistryChemical Kinetics and Nuclear ChemistryNumerical+4 / −1
For a first order reaction A →\to→ B, the rate constant, k = 5.5 ×\times× 10 −-− 14 s −-− 1. The time required for 67% completion of reaction is x ×\times× 10 −-− 1 times the half life of reaction. The value of x is ‾\underline{\hspace{2cm}}​ (Nearest integer) (Given : log 3 = 0.4771)
Numerical answer
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Correct answer: 16

  1. For a first-order reaction, t=2.303klog⁡[A]0[A]t = \frac{2.303}{k}\log\frac{[A]_0}{[A]}t=k2.303​log[A][A]0​​

  2. For 67%67\%67% completion, the reactant left is 33%33\%33%: [A][A]0=0.33=33100\frac{[A]}{[A]_0} = 0.33 = \frac{33}{100}[A]0​[A]​=0.33=10033​ Hence, t67%=2.303klog⁡10033t_{67\%} = \frac{2.303}{k}\log\frac{100}{33}t67%​=k2.303​log33100​

  3. The half-life for a first-order reaction is: t1/2=0.693k=2.303klog⁡2t_{1/2} = \frac{0.693}{k} = \frac{2.303}{k}\log 2t1/2​=k0.693​=k2.303​log2

  4. Therefore, t67%t1/2=log⁡(100/33)log⁡2\frac{t_{67\%}}{t_{1/2}} = \frac{\log(100/33)}{\log 2}t1/2​t67%​​=log2log(100/33)​

  5. Now evaluate log⁡(100/33)\log(100/33)log(100/33): log⁡10033=log⁡100−log⁡33\log\frac{100}{33} = \log 100 - \log 33log33100​=log100−log33 =2−(log⁡3+log⁡11)= 2 - (\log 3 + \log 11)=2−(log3+log11) This is awkward directly, so write: 10033=3.03…\frac{100}{33} = 3.03\ldots33100​=3.03… More neatly, log⁡10033=log⁡100−log⁡33=2−(log⁡3+log⁡11)\log\frac{100}{33} = \log 100 - \log 33 = 2 - (\log 3 + \log 11)log33100​=log100−log33=2−(log3+log11)

    But the standard shortcut is: 10033≈3.03\frac{100}{33} \approx 3.0333100​≈3.03 so log⁡10033≈log⁡3.03≈0.481\log\frac{100}{33} \approx \log 3.03 \approx 0.481log33100​≈log3.03≈0.481

    Also, log⁡2=0.3010\log 2 = 0.3010log2=0.3010

    Hence, t67%t1/2≈0.4810.301≈1.60\frac{t_{67\%}}{t_{1/2}} \approx \frac{0.481}{0.301} \approx 1.60t1/2​t67%​​≈0.3010.481​≈1.60

  6. Given that t67%=x×10−1×t1/2t_{67\%} = x \times 10^{-1} \times t_{1/2}t67%​=x×10−1×t1/2​ so x×10−1=1.60x \times 10^{-1} = 1.60x×10−1=1.60 x=16.0x = 16.0x=16.0

  7. Nearest integer: 16\boxed{16}16​

The derived answer matches the stored correct answer.

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