[maranda101]

My question reads as follows...

An isotope has a half-life of 56 days. It now has 10,000 DPM/mL. What will its activity be 100 days from now?

The reason I am confused about how to solve this is because we were not given the decay constant, nor were we given the name of the specific isotope to look it up.

The next part of the problem says "An isotope now has 5,000 DPM/mL. 60 days ago its activity was 8,000 DPM/mL. What is the half-life of this isotope?

Any guidance or hints would be appreciated. Thanks!

[Corribus]

Half-life is equal to ln(2) divided by the decay constant.

See: http://en.wikipedia.org/wiki/Half-life

It's also an excellent computer game.

[maranda101]

What if we are not given the decay constant or the specific isotope to look it up??

[Corribus]

You are given the half-life.  From that you can determine the decay constant.  You don't need the specific isotope - it's irrelevant.

[maranda101]

nevermind, that was a stupid question...

[maranda101]

thanks!