8.75 g
We are given;
Mass of Os-182 sample = 10.0 g
Number of half lives = 3
Half life = 21.5 hours
We need to determine the mass that decayed after 3 half lives
Using the formula;
Remaining mass = Initial mass × (1/2)^n , where n is the number of half lives
Therefore;
Mass remaining = 10.0 g × (1/2)^3
=1.25 g
Thus, to get the mass that decayed;
Mass that decayed = Initial mass - remaining mass
= 10.0 g - 1.25 g
= 8.75 g
Hence, the mass of Os-182 that decayed is 8.75 g