order of reaction

Question

For a first-order reaction, the half-life is 20 minutes. The time required for 75% completion is:

RankGuru · Accepted Answer

For a **First-Order Reaction**, the half-life ($t_{1/2}$) is constant.
- Let $[A]_0$ be the initial concentration.
- After 1st half-life ($20 	ext{ min}$), $[A] = 50\%$ of $[A]_0$.
- After 2nd half-life (another $20 	ext{ min}$), $[A] = 50\%$ of $50\% = 25\%$ of $[A]_0$.
- **75% completion** means $25\%$ reactant is remaining.
- Total time $= 1$st half-life $+ 2$nd half-life $= 20 + 20 = 40 	ext{ minutes}$.
Alternatively, for first order, $t_{75\%} = 2 	imes t_{50\%}$.

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order of reaction

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