Question

Question #369a9

Answer 1

It will take 24.3 days.

After 1 Half-Life there will be. 25g
After 2 Half-Lives there will be 12.5g
After 3 Half-Lives there will 6.25g

3 Half-Lives = 3 x 8.1 = 24.3 days.

If you don't have such nice numbers you may have to use the general equation for radioactive decay:

`N_t = N_0e^(-lambdat)`

`N_t` is the amount after time `t`

`N_0` is the initial amount

`t` is the time elapsed

`e` is the natural base of logarithms

`lambda` is the decay constant which `=(0.693)/(t_((1)/(2))`

Answer 2

Half-life (t½) is the amount of time required for a quantity to fall to half its value as measured at the beginning of the time period.

In this question (t½) of I-131 is 8.1 days, which means that after 8.1 days half of the sample would have decayed and half would be left as it is.

After 8.1 days ( first half life) 131 /2 = 25g decays and 25 g remains left.

After another 8.1 days ( two half lives or 16.2 days) 25 /2 = 12.5 g decays and 12.5 g remains left .

After another 8.1 days ( three half lives or 24.3 days ) 12.5 /2 = 6.25 g decays and 6.25 g remains left.

after three half lives or 24.3 days, 6.25 g of I-131 will be left.