The half-life of hydrogen-3 is 12.3 years. Given 100 g of hydrogen-3, how many grams will be left after 5
half-lives?

After 5 half life the amount remains as 3.125 g

Explanation:

After each half life the quantity becomes half

so we know that initial amount is 100 g

so after 1st half life amount is half so it left as

m = 50 g

now after 2nd half life amount again becomes half

m = 25 g

after 3rd half life it is again half so remaining amount is

m = 12.5 g

after 4th half life the amount becomes half again

m = 6.25 g

Now after 5th half life the amount remains as

m = 3.125 g

k2s for potassium sulphide