$\mathrm{t}_{87.5}$ is the time required for the reaction to undergo $87.5 \%$ completion and $\mathrm{t}_{50}$ is the time required for the reaction to undergo $50 \%$ completion. The relation between $\mathrm{t}_{87.5}$ and $\mathrm{t}_{50}$ for a first order reaction is $\mathrm{t}_{87.5}=x \times \mathrm{t}_{50}$ The value of $x$ is ___________. (Nearest integer)
Solution
For a first-order reaction, the relation between the reaction rate constant (k) and time (t) for a given percentage of completion (p) is:
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$\ln\left(\frac{1}{1-p}\right) = kt$
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For t₅₀ (50% completion), p = 0.5:
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$\ln\left(\frac{1}{1-0.5}\right) = k\cdot t_{50}$<br/><br/>
$\ln\left(\frac{1}{0.5}\right) = k\cdot t_{50}$<br/><br/>
$\ln(2) = k\cdot t_{50}$
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For t₈₇.₅ (87.5% completion), p = 0.875:
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$\ln\left(\frac{1}{1-0.875}\right) = k\cdot t_{87.5}$<br/><br/>
$\ln\left(\frac{1}{0.125}\right) = k\cdot t_{87.5}$
$\ln(8) = k\cdot t_{87.5}$
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Now, we need to find the relationship between t₈₇.₅ and t₅₀:
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$\frac{k\cdot t_{87.5}}{k\cdot t_{50}} = \frac{\ln(8)}{\ln(2)}$
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Since the k's cancel out, we have:
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$\frac{t_{87.5}}{t_{50}} = \frac{\ln(8)}{\ln(2)}$
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Using the property of logarithms, we get:
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$\frac{t_{87.5}}{t_{50}} = \frac{\ln(2^3)}{\ln(2)}$
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$\frac{t_{87.5}}{t_{50}} = 3$
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So, the value of x is 3.
About this question
Subject: Chemistry · Chapter: Chemical Kinetics · Topic: Rate of Reaction
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