Question:
A first order reaction is 50% completed in 10 minutes. Find the percentage of reactant reacting in 20 minutes.
A first order reaction is 50% completed in 10 minutes. Find the percentage of reactant reacting in 20 minutes.
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A first order reaction has a constant half-life, so 50% completion in 10 minutes means one half-life is 10 minutes.
Updated On: Nov 12, 2026
- \(25\%\)
- \(100\%\)
- \(75\%\)
- \(20\%\)
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The Correct Option is C
Solution and Explanation
Step 1: Understanding the Concept
For a first order reaction the half-life does not depend on the initial concentration. 50% completion in 10 minutes means \(t_{1/2} = 10\) min.
Step 2: Number of half-lives
In 20 minutes the number of half-lives is \(\dfrac{20}{10} = 2\).
Step 3: Calculation
After the first half-life 50% is left, and after the second half-life 25% is left.
Reactant reacted \(= 100 - 25 = 75\%\).
Final Answer:
75% of the reactant reacts in 20 minutes. \[ \boxed{\text{(C) }75\%} \]
For a first order reaction the half-life does not depend on the initial concentration. 50% completion in 10 minutes means \(t_{1/2} = 10\) min.
Step 2: Number of half-lives
In 20 minutes the number of half-lives is \(\dfrac{20}{10} = 2\).
Step 3: Calculation
After the first half-life 50% is left, and after the second half-life 25% is left.
Reactant reacted \(= 100 - 25 = 75\%\).
Final Answer:
75% of the reactant reacts in 20 minutes. \[ \boxed{\text{(C) }75\%} \]
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