Half-Life Calculations

If 10 mg of iodine 131 is given to a patient, how much is left after 24 days? The half-life of iodine-131 is 8 days.

• 1.25mg
• 1.25g
• 10g
• 10mg

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NGSS.HS-PS1-8

The half-life of strontium-90 is 25 years. How much strontium-90 will remain after 100 years if the initial amount is 4.0 g?

• 3.0g
• 0.25mg
• 0.3g
• 0.25g

Tags

NGSS.HS-PS1-8

If the half-life of uranium-232 is 70 years, how many half-lives will it take for 10 g of it to be reduced to 1.25 g?

• 1 half-life
• 2 half-lives
• 3 half-lives
• 4 half-lives

Tags

NGSS.HS-PS1-8

After 22,800 years, approximately what percentage of the original carbon-14 remains?

• 15%

• 12.5%

• 6.25%

• 3.125%

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NGSS.HS-PS1-8

The % of the parent isotope remaining after 1 Half Life.

• 50%

• 25%

• 12.5%

• 6.25%

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NGSS.HS-PS1-8

The % of the parent isotope remaining after 2 Half Lives.

• 50%

• 25%

• 12.5%

• 6.25%

Tags

NGSS.HS-PS1-8

The mass of a radioisotope falls from 1.6g to 0.1g in 2 hours. What is the half-life of this radioisotope (in minutes)?

Answer explanation

Our first step is to work out how many times the mass halves. After the first time there would be 1.6/2 = 0.8g left, after the second there would be 0.8/2=0.4g left, the third leaves 0.4g/2=0.2g and then the fourth would leave us with 0.1g.

 

Then we divide our time by our number of half-lives. 2 hours/4 half-lives, so the half-life of this sample is half an hour. 

Tags

NGSS.HS-PS1-8

If a 1g sample of a radioisotope with a half-life of 3 days has an activity of 32 counts sec^-1, how long would it take for the activity of a 2g sample to fall to 8 counts sec^-1?

Give your answer in days

Answer explanation

Our first step is to work out what the starting activity would be. The activity of 1g was 32 counts per second (cps) so the activity of 2g will be 64 cps.

Then we calculate our number of half-lives. 64 > 32 > 16 > 8. So, we have three half-lives.

Then we just multiply the length of each half-life by the total number of half-lives. 3 days*3 half-lives tell us it would take 9 days for the activity to fall to 8 cps.    

Tags

NGSS.HS-PS1-8

A medical radioisotope has a half-life of 1.5 hours. Its radioactivity was recorded as 10,000 counts min^-1 at 9am. What will the count rate be by 3pm?

Answer explanation

Our first step is working out that there are 6 hours between 9am and 3pm. If the half-life is 1.5 hours, then there are 4 half-lives over this time period. Then, we half the starting value four times to get our final answer.

10,000 > 5,000 > 2,500 > 1,250 > 625 counts per minute.

Tags

NGSS.HS-PS1-8

Phosphorus-32 is a radioisotope with a half-life of 14.3 days. If you start with 4g of phosphorus-32, how many half-lives will have passed after 57.2 days? 

Answer explanation

All we have to do for this calculation is divide the total time by the length of each half life. 57.2 / 14.3 = 4 half-lives.

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NGSS.HS-PS1-8