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A first order reaction is half completed in `45` minutes. How long does it need `99.9%` of the reaction to be completedA. `5` hoursB. `7.5` hoursC. `10` hoursD. `20` hours

Answer» Correct Answer - B
`k = (0.693)/(45)"min"^(-1) = (2.303)/(t_(99.9%))"log"(a)/(a-0.999a)`
or `t_(99.9%) = (2.303xx45)/(0.693)log10^(3) = 448 min ~~ 7.5 hrs`


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