Question

Half life for a first order reaction is 693 seconds calculate the time required for 90% completion of this reaction.

Answer 1

Let the half life is Denoted by T

and initial amount of substance be A

and the disintegration constant by k

we know that that the equation is

kT = .693  [i.e ㏑2 = .693]

from here k = .693/693 = .001

now after 90% completion only 10% of original substance will be left

[i.e .1*A = a ]

applying the formula ㏑[A/a] =kt

we get ㏑[A/.1*A]=2.303 = kt

therefore 2.303/k  = t

that is 2303 seconds

38.3 min