chemistry please help?

Question

half-life of phosphorous-33 is 25.3 days. how long will it take for 64 grams of p-33 to decay to 1.0 grams? 10.67days, or 75days, or 100days, or 120days, or 150days

Answer 1

if 1 gm is left of 50 gms then the fraction of mass left = 1/64
= 1/2^6 = (1/2)^6

it clearly indicates that, the substance need 6 half lives to reduce from 64 gm to 1 gm.

so the time reqd. = 6 * 25.3 days.
=151.8days.

the last option is the nearest.

Answer 2

You have to halve 64 six times to get down to 1, so it takes 6 x 25.3 = 151.8 days. So the answer is "150 days."