The strontium-90 isotope decays by the reaction below. It has a half-life of 28 yr. If the initial activity of this isotope was 168dpm, what would the activity be after 28.0 yr?

This is done the same way as the Sr-87.

To determine the activity of the strontium-90 isotope after 28.0 years, we can use the concept of radioactive decay and the half-life of the isotope.

The activity of a radioactive substance is a measure of how many radioactive decays occur per unit time. It is typically measured in disintegrations per minute (dpm).

The half-life of an isotope is the amount of time it takes for half of the radioactive atoms to decay. In this case, the half-life of strontium-90 is 28 years.

To calculate the activity after a certain time, we can use the following equation:

Activity Final = Activity Initial * (1/2)^(Time / Half-life)

Given that the initial activity is 168 dpm and the time is 28.0 years (which is equal to the half-life), we can substitute these values into the equation:

Activity Final = 168 * (1/2)^(28.0 / 28)

Simplifying this calculation:

Activity Final = 168 * (1/2)^1

Using the exponential expression:

Activity Final = 168 * 0.5

Activity Final = 84 dpm

Therefore, the activity of the strontium-90 isotope after 28.0 years would be 84 dpm.