Determine the total time required for an 80.0-gram sampleof I-131 to decay until only 1.25 grams of the sample remains unchanged.

481.2 days

Explanation:

Initial mass of I-131 is 80.0 gram

Remaining mass of I-131 after decay is 1.25 grams

We are required to determine the time taken for it to decay;

The half-life of I-131 is 80.2 days

But;

Remaining mass = Initial mass * (1/2) ^n, where n is the number of half lives

Therefore;

1.25 g = 80.0 g * 0.5^n

0.015625 = 0.5^n

n = log 0.015625 : log 0.5

= 6 half lives

But, Time = half life * number of half lives

= 6 * 80.2 days

= 481.2 days

Therefore, it will take 481.2 days for a mass of 80.0 g I-131 to decay to 1.25 g.