PD-99 has a half-life of 22.3 hours. How many grams of a 89.8 gram sample would have decayed after exactly three half-lives?

78.875 g

Explanation:

First half life is the time at which the concentration of the reactant reduced to half.

Second half reaction is the time at which the remaining concentration reduced to half or the initial concentration reduced to 1/4.

Third half life is the time at which the remaining concentration reduced to half or the initial concentration reduced to 1/8.

The initial mass of the sample = 89.8 g

After 3 half-lives, the mass should reduce to 1/8.

So the concentration left = 1/8 of 89.8 g = 11.225 g

So, the mass of the sample decayed = 89.8 g - 11.225 g = 78.875 g