Answer:
e) 400 mg of a radioactive substance with a half-life of 8 hours after decaying for 8 hours
a) 800 mg of a radioactive substance with a half-life of 2 years after decaying for 4 years.
Explanation:
a) 800 mg of a radioactive substance with a half-life of 2 years after decaying for 4 years.
Number of half lives passed = Time elapsed/ half life
Number of half lives passed = 4 year/2 year
Number of half lives passed = 2
at time zero = 800 mg
At first half life = 800 mg/2 = 400 mg
At 2nd half life = 400 mg/2 = 200 mg
b) 1,000 mg of a radioactive substance with a half-life of 5 years after decaying for 20 years
Number of half lives passed = Time elapsed/ half life
Number of half lives passed = 20 year/ 5 year
Number of half lives passed = 4
at time zero = 1000 mg
At first half life = 1000 mg/2 = 500 mg
At 2nd half life = 500 mg/2 = 250 mg
At 3rd half life = 250 mg/2 = 125 mg
At 4th half life = 125 mg/2 = 62.5 mg
c) 300 mg of a radioactive substance with a half-life of 1 year after decaying for 1 year
Number of half lives passed = Time elapsed/ half life
Number of half lives passed = 1 year/ 1 year
Number of half lives passed = 1
at time zero = 300 mg
At first half life = 300 mg/2 = 150 mg
d) 600 mg of a radioactive substance with a half-life of 6 hours after decaying for 18 hours
Number of half lives passed = Time elapsed/ half life
Number of half lives passed = 18 hours / 6 hours
Number of half lives passed = 3
at time zero = 600 mg
At first half life = 600 mg/2 = 300mg
At 2nd half life = 300 mg/2 = 150 mg
At 3rd half life = 150 mg/2 = 75 mg
e) 400 mg of a radioactive substance with a half-life of 8 hours after decaying for 8 hours
Number of half lives passed = Time elapsed/ half life
Number of half lives passed = 8 hours / 8 hours
Number of half lives passed = 1
at time zero = 400 mg
At first half life = 400 mg/2 = 200 mg
