1.

A first order reaction is 50% completed in 1.26xx10^(14)S.How much time would it take for 100% completion?

Answer»

`1.26xx10^(15)s`
`2.52xx10^(14)s`
`2.52xx10^(28)s`
infinite

Solution :Whole of the substance never reacts because in every HALF life.505 of the substance reacts.Hence ,TIME taken for 100% completion of a reaction is infinite.
`t_((1)/(2))=1.26xx10^(4)s`
`therefore k=(0.693)/(t^((1)/(v)))=(0.693)/(1.26xx10^(4))`
The FINAL concentration when the reaction is completed 100% =`[R]_(t)`=zero
So `t_(100%)=(2.303)/(k)` log `([R]_(0))/([R]_(t))` `(2.303)/(k)` log (infinite)
`~~(2.303)/(k)xx10^(prop)=prop`
If any reaction `t_((1)/(2))` =exact value ,then it will take infinite time for 100% completion .So option (D) is correct.


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