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Time for a given fraction of a first order reaction, or the amount left, from its rate constant

This type of question, asked 10 times: 2 May 2023 S1, 9 May 2024 S1, 12 May 2024 S1, 23 Apr 2025 S1, 25 Apr 2025 S2, 11 Apr 2026 S1, 18 Apr 2026 S2, 12 May 2026 S2, 13 May 2026 S1, 19 May 2026 S2. All 11 questions below, oldest first.

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  1. Question 1

    2 May 2023 S1

    63
    Calculate the time required for reactant to decrease the concentration from 100% to 20%, if rate constant of first order reaction is 0.02303 hours−1^{-1}.

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  2. Question 2

    9 May 2024 S1

    90
    Calculate the amount of reactant in percent that remains after 60 minutes involved in first order reaction (k=0.02303 min−1k=0.02303\,min^{-1})

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  3. Question 3

    12 May 2024 S1

    63
    Calculate the time needed for reactant to decompose 99.9% if rate constant of first order reaction is 0.576 minute−10.576\,\text{minute}^{-1}.

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  4. Question 4

    23 Apr 2025 S1

    68
    The rate constant of a first order reaction is 1.15×10−3 s−11.15 \times 10^{-3}\,\text{s}^{-1}. How long will 5 g of reactant take to reduce to 3 g?

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  5. Question 5

    25 Apr 2025 S2

    83
    What is the time required for 99%99\% completion of a first order reaction if rate constant is 23.03 min⁡−123.03{\text{ }\min}^{- 1} ?

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  6. Question 6

    11 Apr 2026 S1

    28
    The rate constant for a first order reaction is 60 s−160\ \mathrm{s^{-1}}. How much time will it take to reduce the concentration of the reactant to 1/20th1/20^{th} of its initial value?

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  7. Question 7

    18 Apr 2026 S2

    28
    What is the time required for 75 % completion of a first order reaction if rate constant is 23.03 minute−123.03\ \mathrm{minute^{-1}} ?

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  8. Question 8

    18 Apr 2026 S2

    29
    For a reaction A⟶product\mathrm{A \longrightarrow product}, k=2×10−2 s−1k = 2 \times 10^{-2}\ \mathrm{s^{-1}}. If the initial concentration of A is 1.0 mol dm−31.0\ \mathrm{mol\,dm^{-3}} find the value of log⁡1[A]t\log\dfrac{1}{[A]_t} after 100 second ?

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  9. Question 9

    12 May 2026 S2

    27
    Rate constant for first order reaction is 40 s−140\,s^{-1}. Calculate the time required to reduce the concentration to 1/201/20 th of its initial value.

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  10. Question 10

    13 May 2026 S1

    27
    The rate constant for a first order reaction is 4.606×10−3 s−14.606 \times 10^{-3}\ \mathrm{s^{-1}}. Calculate the time required to reduce 2.02.0 g of the reactant to 0.20.2 g .

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  11. Question 11

    19 May 2026 S2

    28
    The rate constant for a first order reaction is 2.2×10−5 s−12.2 \times 10^{-5}\ \mathrm{s^{-1}} at 320∘C320^\circ\mathrm{C}. Calculate the time required for 15%15\% completion?

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