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There is a few main things to remember

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There is a few main things to remember. 1) The radioactive half-life is the time taken for half the starting material to disappear. [Starting Amount] ##*## ##[1/2]^n## = Final Amount Where n is the number of half lives 2) The number of half-lives can be expressed as n = total time / length of half-life)

For example: Lets say the half life of a radioactive substance is 30minutes. If there is 100g of starting material, how much is left after 2 hours.

Answer: Recognise 2 hours = 4 x 30 minutes, so n=4 half lives [100g] ##*## ##[1/2]^4## = 6.25g

Worked examples can be found here: ChemTeam Half-Life Problems