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Half-life problems

Nuclear chemistry is the last topic students learn in their chemistry classes. It is an important topic for the chemistry regent. The two sections in nuclear chemistry that have the most questions on the regent exam are balancing nuclear equations and doing half life problems.

Half life is the amount of time it takes for the substance to become one half of its initial value. For example, if substance x has a half life one day and starts out as 100 grams, in one day, there will be 50 grams of it is left.

On the Reference table, students can find different radioisotopes with their respective half lives and modes of decay that can be found on Table N. Therefore, if the question sounds like a half life questions but doesn't give half-life, most likely the student is expected to go to the reference table to find the half life.

Let's look at some of our example to see how to solve half life problems?

Example 1: If 50 grams of Cs-137 are disintegrated to 6.25 grams, how much time has passed?

Solution: 50g ------> 25g -------> 12.5 -------> 6.25g

Each arrow represents a half life period. Therefore, three half lives have passed for 50g to become 6.25g. Looking at the Reference Table, we see that the half life for Cs-137 30.2 years. Thus, 30.2 years*3 = 90.6 years must have passed.

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