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Chemistry Regents: How to Perform Half-Life Calculations

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In this video, I will show you how to perform Half-Life Calculations. Half-life is defined as the amount of time needed to get half as much, particularly for radioisotopes, which can be found in Table N of the Chemistry Reference Tables. The time frame for half life can be measured in years, days, hours, minutes, seconds and even milliseconds. Let’s see how this works using carbon 14. If we start out with a 100 gram sample of carbon-14, we’ll call this the original amount. The amount of time needed to get half as much, or 50 grams, referred to as the amount remaining, is 5715 years, which is the half life of carbon-14. If another 5,715 years were to go by, the amount remaining would now be 25 grams, which is half of 50. Carbon-14 will continue decaying at this rate so that the amount remaining will always be half of the previous amount and could go on forever! So how do you know when to stop? Well, the good news is that Chemistry Regents will only go as far as 5 half life periods, which is the number of times a radioisotope gets cut in half.

Another way to arrive at the whole numbers of 1, 2, 3, 4 or 5 is by dividing the Total time that passed by the half-life. Let’s see how this works using the following chemistry regents question. What is the total number of years, or the total time, that must pass before only 25 grams, like in our example, of an original 100 gram sample of Carbon 14 remains unchanged. Since we started with 100g and ended with 25g, only 2 half life periods went by.

We also know that Carbon 14’s half life is 5715 years. So, what, divided by 5715 equals 2? Simply, cross multiply and you will get your answer. Now before I go ahead and give you a few practice problems, there is one other relationship that you should be aware of and that is for the fraction remaining. 50 grams divided by the original amount of 100 is equivalent to 1/2, or 1 half life period. 25 grams divided by 100 is equivalent to 1/4, which corresponds to 2 half life periods. Each successive mass, when divided by the original amount of 100 grams will reduce to the fraction remaining that corresponds to the number of half life periods. Please take a minute to like this video if you found it helpful. Your comments are appreciated as well. Here are 3 Half-life questions for you to try on your own.

When you are done, click the link below to check your answers. Good Luck and thank you for watching!.

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