Answer:
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.
