Question

In which scenarios would the amount of substance remaining be 200 mg? Select all that apply. 800 mg of a radioactive substance with a half-life of 2 years after decaying for 4 years 1,000 mg of a radioactive substance with a half-life of 5 years after decaying for 20 years 300 mg of a radioactive substance with a half-life of 1 year after decaying for 1 year 600 mg of a radioactive substance with a half-life of 6 hours after decaying for 18 hours 400 mg of a radioactive substance with a half-life of 8 hours after decaying for 8 hours

Answer 1

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