5.

A 120.0 g sample of Thallium-208 decays. How much of the sample will remain after 3 half lifest

15.0 g

Explanation:

We are given;

Original sample of Thallium-208 as 120.0 g The number of half lives as 3

We are required to calculate the remaining mass after three half lives

We are going to use the formula;

N = N₀ * 0.5^n

where, N is the remaining mass, N₀ is the original mass and n is the number of half lives.

Therefore;

N = 120.0 g * 0.5^3

= 15.0 g

Thus, the remaining mass of Thallium-208 is 15.0 g

Alternatively;

Since after every half life the mass reduces by half of the original then we can half a chain showing the reduction after three half lives;

120.0 g → 60.0 g (120 * 0.5) → 30.0 g (60 * 0.5) → 15.0 g (30 * 0.5)

Therefore, the remaining mass after three half lives is 15.0 g