If 20. g of a radioactive isotope are present at 1:00 pm and 5.0g remain at 2:00 pm, what is the half life of the isotope?

The half life of the isotope is calculated as follows

the time difference = 2.00 - 1.00 = 1 hour (60min)

if 20 g decay in 1 hour (60min) and 5 grams remains then the mass that decay = 20-5 = 15 grams

the half life is time required for a quantity to reduce to half its initial value

and if the initial concentration = 20g then the half = 20/2 = 10g

then time = 10g x 60min/15g = 40 min