Technetium-104 has a half-life of 18.0 minutes. How much of a 165 g sample is left after 90.0 minutes?

After 90 minutes 5.15625 g amount left from total of 165 g.

Explanation:

Given data:

Technetium-104 half life = 18.0 min

Total amount of sample = 165 g

Amount left after 90.0 min = ?

Solution:

First of all we will calculate the number of half lives passes.

Number of half lives = T elapsed / Half life

Number of half lives = 90 min /18.0 min

Number of half lives =  5

Amount left:

At time 0 = 165 g

At first half life = 165 g/ 2= 82.5 g

At 2nd half life = 82.5 g/2  = 41.2 g

At 3rd half life = 41.2 g/ 2 = 20.625 g

At 4th half life =  20.625 g/ 2  = 10.3125 g

At 5th half life = 10.3125 g /2 = 5.15625 g

Thus, after 90 minutes 5.15625 g amount left from total of 165 g.