How long would it take for the concentration of the reactant to drop to one-forurth its initial value?

Question

initial concentration of reactant=0.64mol/L
half-life=30s

Answer 1

0.64 to 1/4(0.64) = 0.16
0.64/0.16 = 4 so it must go through 2^n = 4 which makes n = 2 half lives.
If the half life is 30 sec, then it will take 60 seconds for it to go through 2 half lives.