Question

For a particular first-order reaction, it takes 3.0 minutes for the concentration of the reactant to decrease to 25% of its initial value. what is the value for rate constant (in s-1) for the reaction?

Answer 1

We will use this formula for first order:
㏑[A] = - Kt +Ao
when we have t (given)= 30 min = 30 x 60 = 1800 s (we here convert time from min to second.
then we assume that the initial concentration Ao = 1
and the concentration of A (final concentration = 0.25 
So by substitution:
㏑(0.25) = - K * 1800 + ㏑(1)
1.39 = K * 1800 
∴ K = 0.00077 s^-1 or 7.7 x 10^-4