1. The half-life of 9142Mo is 15.5 minutes. If you start with 3.5 x 1022 atoms of 9142Mo, how many atoms will remain after 31 minutes?

2. The half-life of 31H is 12.26 years. If you start with 2.0 x 1015 atoms of 31H, how many atoms will remain after 36.78 years?

3. The half-life of 6932Ge is 30 hours. If you start with 3.0 X 105 atoms of 6932Ge, how many atoms will remain after 120 hours?

4. The half-life of 146C is 5730 years. If you start with 2.5 X 109 atoms of 146C, how many atoms will remain after 28,650 years?

5. The half-life of 198O is 29.1 seconds. If you start with 1.5 x 1022 atoms of 198O, how many atoms will remain after 4 minutes and 51 seconds?

Per the homework policy of ask me help desk, we can't just give you the answer. What we can do is to help you where you are having difficulty if you will show us the answers you got and tell us what you don't understand. These are pretty simple... do you know the formula for figuring out the half life?

The half-life of an element is the time it takes for 1/2 of the atoms to decay. For example: if the element's half life is 30 seconds then after 30 seconds half the original atoms of the element remains. After another 30 seconds (60 seconds total) 1/2 of what was left at 30 seconds remains, so you have 1/4 of the original. After 90 seconds you are left with 1/8 of the original, after 120 seconds you have 1/16, etc.

For the first 4 problems you just need to divide the starting number of atoms by 2 enough times to equal the number of half-life times. That method works as long as the number of half-life periods is a whole number. But in the last problem the number of half life times is not a whole number (29.1 doesn't divide evenly into 51) so you have to use the formal equation:


N(t) =N_0 ( \frac 1 2 )^{{(t/H)}


where N_0 is the number of atoms you started with and H = the element's half-life.

Post back with your answers and we'll check it for you.