Calculating half lives?

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How many half-lives are needed for a 400.0 ng sample of a radioactive isotope to decay to 12.5ng?

2006-11-28 04:19:15 · 3 answers · asked by carmond70 1 in Science & Mathematics ➔ Chemistry

5 Half-Lives.

After 1: 200ng
After 2: 100ng
After 3: 50ng
After 4: 25ng
After 5: 12.5ng

2006-11-28 04:22:02 · answer #1 · answered by Stu 2 · 0⤊ 0⤋

Five. 1/2 X 1/2 X 1/2 X 1/2 X 1/2 = 1/32

1/32 X 400 = 12.5

2006-11-28 04:31:27 · answer #2 · answered by JIMBO 4 · 0⤊ 0⤋

400/2 = 200 /2 = 100/2 = 50/2= 25/2 = 12.5
now count the No. of divisions and the answer is 5

2006-11-28 04:53:20 · answer #3 · answered by Jan Zam 1 · 0⤊ 0⤋