Question

The half-life of radon-222 is 3.8 days. If a sample currently contains 3.1 grams of radon-222, how much radon-222 did this sample have 15.2 days ago?
a 12.4 grams
b 47.1 grams
c 49.6 grams
d 57.8 grams

Answer 1

The   number  of grams of  radon 222 did it have 15.2  ago was  49.6 grams( answer  C)

 calculation

• calculate the number of half life it has covered    from   15.2  days to 3.8 days

             that is  divide 15.2/ 3.8 = 4 half life

•  half  life is time taken for a radio activity  of a specified isotope to fall to half its original mass

therefore  3.8 days ago  it was  3.1 x2 =  6.2 grams

                  7.6 days ago  it was 6.2 x2 = 12.4 grams

                   11.4  days  ago it was 12.4  x2=  24.8  grams

                  15.2  days  ago  it was  24.8 x2=49.6 grams