Posted by **Jazmin** on Sunday, May 15, 2011 at 8:29pm.

K-42 has a half-life of 12.4 hours. If you begin with 250g of

K-42, determine how much you will have left after 4 half-lives.

How long does 4 half-lives take?

please help

- Chemistry -
**DrBob222**, Sunday, May 15, 2011 at 8:46pm
There is a shorter way, I think, but this is what I would do.

k = 0.693/t_{1/2}

Substitute k into the equation below.

ln(No/N) = kt

No = 250

N = unknown

k from above.

t = time = 4*12.4 = ??

The short way is (n = number of half lives)

1/2^n = fraction left

1/2^4 = ?? and ?? x 250 = xx.

You should get the same answer either way.

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