# Phosporous-32 is a radioisotope used in molecular biology After 44 days, 17% of the sample remains. How do you calculate the half-life?

##### 1 Answer

Dec 5, 2016

You can do it like this:

#### Explanation:

The expression for 1st order decay is:

The graph looks like this:

The key to the problem is to find the decay constant

Taking natural logs of both sides gives:

If we assume we start with 100 undecayed atoms this becomes:

This is related to the 1/2 life by: