What formula would I use for this?

Question

The radioactive isotope californium-253 has a half-life of 18 days. If you begin with 60 grams of this isotope, how much is remaining after 90 days?

Answer 1

You are able to use the formula below or you can:

1. divide 90 days by 18 days (days of half life) to get # of half-life = 5
2. (1/2) ⁵ = 0.03125 or 3.125 % - if you struggle to calculate this (without calculator) then do this:
3. 1 half life = 100/2 = 50% left
4. 2 half life = 50/2 = 25 % left
5. 100 -> 50 -> 25 -> 12.5 -> 6.25 -> 3.125% (the number of arrows is the number of half-life)
6. therefore 5 half-lifes = 3.125% left then go to #3 below.
7. or divide 60g by 2 - 5 times (5 half lives) ex 60g - 30g -15g -7.5g -3.75 g - 1.875g
8. Then multiply 60g by 3.125% = 1.875 g

	=	quantity of the substance remaining
	=	initial quantity of the substance
	=	time elapsed
	=	half life of the substance