Question

calculate the half-life of a first- order reaction if the concentration of the reactant is 0.0396 M at 12 seconds after the reaction starts and is 0.00579 M at 47 after the reaction starts. How long does it take for the reactant concentration to decrease to 0.00269 M

Answer 1

ln(No/N) = kt
No = 0.0396
N = 0.00579
k = solve
t = 47-12 = 35 seconds.
Solve for k, then
k = 0.693/t_1/2 and solve for t_1/2

Then
ln(No/N) = kt and plug in the new numbers to find time for the 0.00269M to be reached.