Percentage of Substance Reacted

[Suzan Manoian 1H]

How do we calculate the half-life of a first order reaction if the problem lists the percentage a substance has reacted? In other words, if it says 15% of the substance reacted, without giving the initial concentration?

[Darya Pahlavan]

For first order reactions, you dont need the concentration to determine the half life. The equation is t1/2= .693/k. For the type of problem you are talking about maybe try using ln([A]/[A]0) = -Kt using the ratio of the percentage that you are given.

[Lexi Brownsberger 2E]

If you are only given the percentage that is reacted, you must determine what the value of k is to then be able to find the half-life of the concentration. For first order, no actual concentration value is needed to determine the half-life, unlike with zero and second order.

[gary_vernes]

I think you can just express both as percentages. Like the intial concentration would be 15%, and the concentration at time t would just be 100 - 15.