Half-life question.?

Question

What is the amount of time it takes for three-fourths of a radioactive sample of an isotope of bromine to decay? The half-life of that isotope is 16.5 hours.

Also, how do I find the half-life for future reference?

Answer 1

i remember these! ok, half of a half is a quarter right? well, if the half life is 16.5 hours, then u take half of that which is 8.25 and add it. meaning u added a quarter to that half to make 3/4 which is wat u want. so ur answer is 24.75 hours.

Answer 2

Sorry, your other answerer blew it. Suppose you have B atoms of bromine at the start of a half-life interval. At the end of that interval, you will have
B/2 atoms left. Suppose at that instant, you start another half-life interval. At the end of the second interval, you have B/4 atoms left. This is your problem, so the answer is two half-lives or 33 hours.

The Handbook of Chemistry and Physics has half-lives for most isotopes of interest. They range over a wide range of time, from fractions of a second to billions of years.