Answer to Question #132833 in General Chemistry for mulweli

Question

The initial concentration of a reactant in a 1st order reaction is 1.20 mol dm-3.
How many half-lives need to elapse for the concentration to drop to 0.075 mols dm-3

Answer 1

Solution:

$\dfrac{1.20 ( \tfrac{mol}{dm^{3}}) }{0.075 (\tfrac{mol}{dm^{3}}) }=16$

So, the final concentration is $\tfrac{1}{16}$ of the original concentration. 4 half-lives are required to reduce the original concentration to $\tfrac{1}{16} th$ of that.