Asample of radiosodium () has a half-life of 15 hr. if the sampleâs activity is 100 millicuries after 24 hr, approximately what must its original activity have been?

Answer : The original activity will be, 303 millicuries.

Explanation :

Half-life = 15 hr

First we have to calculate the rate constant, we use the formula :

Now we have to calculate the time passed.

Expression for rate law for first order kinetics is given by:

where,

k = rate constant  =

t = time passed by the sample  = 24 hr

a = initial amount of the reactant  = ?

a - x = amount left after decay process = 100 millicuries

Now put all the given values in above equation, we get

Therefore, the original activity will be, 303 millicuries.

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