First order half-life

[Marconis]

The radioactive isotope tritium decays with a first-order rate constant k of 0.056 year-1. What fraction of the tritium initially in a sample is still present 30 years later?

 

By using the formula .693/K, I get the answer 12.375 years is the half life. I am honestly stumped as to what is next. Any guidance would be greatly appreciated. Thanks

Edited February 10, 2010 by Marconis

[Cap'n Refsmmat]

What kind of class is this for? There are several ways to solve it, and which you use depends on what mathematical knowledge you have. Also, if you have formulas given to you, rather than being expected to derive them yourself, they could be important.

[Marconis]

This is for Gen Chem 2. My mathematical knowledge is pretty slim, lol.

[Mr Skeptic]

Well, there's two ways to go about this. One is as you did, calculate the half life and then see how many times 1/2 your sample will decay. The other, is to just plug the time into the equation for exponential decay and get your result directly.

[Marconis]

I just found a way to do it using inverse log. Thanks guys

[dttom]

ln(A0/A) = kt

substitute all the number you get the answer.