Question

How long will it take a sample of molybdenum-99 (half-life 67 hours) to decay to 25% of its original activity?

Answer 1

Half life period is the time required to a radioactive compound to reduce it to its half value.

t1/2 = 67 hrs

25% of original amount means 1/4 th of the initial amount.

suppose 100 gm molybdenum-99 compound is present, after 67 hrs, compound will reduce to 50 gm

Further 50 gm of compound will undergo decay to its half value in 67 hrs.

In two half cycles compound will be reduced to 1/4 th of its initial amount

hence molybdenum will decay to its 25% in (67+67 = 134 hrs)