Calculating Half-Life

Half-Life Calculations and Predictions

​Half- life is the length of time it takes for half of a radioactive atoms of a specific isotope to decay.

​When an isotope goes through 1 half-life, 1/2 of the isotope or 50% will remain.

​When an isotope goes through two half-lives, 1/4 or 25% remains.

Multiple Choice

When an isotope goes through 3 half-lives, _____ _________________ remains

• 1/32 of 3.125%

• 1/8 or 12.5%

• 1/16 or 6.25%

​Answer:

Multiple Choice

When an isotope goes through 4 half-lives, _____ _________________ remains

• 1/4 or 25%

• 1/8 or 12.5%

• 1/16 or 6.25%

​Answer:

Multiple Choice

When an isotope goes through 2 half-lives, _____ _________________ remains

• 1/4 or 25%

• 1/2 or 50%

• 1/8 or 12.5%

Answer:

Answer:10 g

​Each half-life adds 5.3 years to the total years.

​The mass will decrease by half with each half-life.

Multiple Choice

How much of an 80 g sample will remain after 21.2 years?

• 40 g

• 20 g

• 10 g

• 5 g

​Answer: 5 g

Multiple Choice

How many years will it take for an 80 g sample to decay to 10 g? (Remember, you must add 5.3 years for each half-life.)

• 10.6 years

• 15.9 years

• 21.2 years

• 26.5 years

​Answer: 15.9 years

​How many years will it take for an 80g sample to decay to 10 grams?

Drag and Drop

If 20 g of copper-64 has a half-life of 13 hours, what mass of copper-64 will remain after 39 hours? ( Drag the answer to the space.)​ ​

Placeholder

Drag these tiles and drop them in the correct blank above

2.5 g

20 g

10 g

1.25 g

5 g

0.625

​Answer:

Multiple Choice

If 20 g of copper-64 has a half-life of 13 hours, what mass of copper-64 will remain after 26 hours?

• 20 g

• 10 g

• 5 g

• 2.5 g

​Answer:

​Interpreting Half-life with Graphs

​The graph produced by radioactive decay is called exponential decay. The graph never really reaches zero.

After 1 half-life, 50% of the substance remains.

​The percent of the sample remaining after 2 half-lives is 25%.

​Determining Half-Life from a Graph

Multiple Choice

What is the half-life of radium-226?

• 1600 years

• 3200 years

• 4800 years

• 6400 years

​Answer: 1600 years

Multiple Choice

What is the half-life of strontium-90?

• 25 days

• 50 days

• 75 days

• 100 days

​Answer: 25 days

​100 g decays to 50 g which is 50%, which is also 1/2 of 100 g.

Multiple Choice

How many grams will remain after 100 g of cobalt-60 has gone through 3 half-lives?

• 50 g

• 25 g

• 12.5 g

• 6.25 g

​How many grams will remain after 100 g of cobalt-60 has gone through 3 half-lives?

Answer: 12.5 g

Multiple Choice

How many years will it take for 75% of 100 g of cobalt-60 to decay?

• 5 years

• 10 years

• 15 years

• 20 years

​How many years will it take for 75% of 100 g of cobalt-60 to decay?

Answer: 10 years ( If 75% decays, only 25% remains.)

​End