Chemistry need answers please

Question

A certain reaction is first order. Determine the amount of time required for the reactant to go from 0.500 M to 0.250 M if the rate constant is 1200. s −1

Answer 1

For a 1st order reaction half life (t1/2) = 0.693/k = 0.693/1200s^-1 = 5.78x10^-4 s

For a 1st order reaction, you can use the simple formula FR = 0.5ⁿ

FR = fraction remaining = 0.250 / 0.500 = 0.5

n = number of half lives that have elapsed

Solving for n we have...

0.5 = 0.5ⁿ

and n = 1 half life = 5.78x10^-4 s