Question

Half life worksheet answer key 1. How long does it take a 100.00g sample of As-81 to decay to 6.25g?

Answer 1

132 seconds ( 2 Minutes 12 seconds).

Step-by-step explanation:

Half life is basically the time it takes for a compound to decay to half of it's original mass.

In this problem, the compound is As. The half life of As-81 is 33 seconds. This means it takes 33 seconds for 100 g of As-81 to decay to 50g. The question however id to find the time it takes for it to decay to 6.25g.

We find how many half life's it would take,

100 --> 50 (First half life)

50 --> 25 (Second half life)

25 --> 12.5 (Third half life)

12.5 --> 6.25 (Fourth half life)

This means the total time is 4 * 33 (Half life) = 132 seconds ( 2 Minutes 12 seconds).

Answer 2

The half life of As-81 is 33 seconds;
6.25 g = 100 × (1/2)^n, where n is the number of half lives
(1/2)^n = 0.0625
n = 4
Thus, the time taken = 33 ×4 = 132 seconds
Therefore, it takes 132 seconds or 2 mins 12 seconds for it to decay to 6.25 g