Posted by Derren on .
The initial concentration of the a substance was 1.95 M. After 27.72 hours the concentration was 0.9750 M. After anothers 55.44 hours the concentration was 0.4875 M (total time = 83.16 hours).
If another experiment were conducted in which the initial concentration was 0.427 M, how long would it take for the substance to reach a concentration 0.0213 M?

chemistry 
DrBob222,
You use 1st order equation of
ln(No/N) = kt and solve for k.
Use the second time period to calculate k again. If k is the same, then this is a first order reaction and you can use that equation on the other experiment. If k is not the same; you assume it is a second order equation and try that for both sets to see if k really is k. If it is then you can use second order equation for the other experiment.
Second order equation is
(1/A)  (1/Ao) = akt.
Post your work if you get stuck.