Technetium has a half-life of six hours. If you obtain 100g of a pure technetium sample, after 18 hours how much of the pure substance will be remaining?

12.5 grams.

Explanation:

You can find the amount of pure substance remaining by using the half-life formula:

Where t is the amount of time, and t2 is the half life of the substance.

Substituting our values for the variables, we get this equation:

After 18 hours, there will be 12.5 grams of the substance remaining.