The isomerization reaction, ch3nc → ch3cn, is first order and the rate constant is equal to 0.46 s-1 at 600 k. what is the concentration of ch3nc after 0.20 minutes if the initial concentration is 0.30 m?

According to this formula :
㏑[A] /[Ao] = - Kt 
when we have Ao = 0.3 m 
and K =0.46 s^-1
t = 20min = 0.2 x 60 =12 s
So by substitution :
㏑[A] / 0.3 = - 0.46 * 12
㏑[A] / 0.3 = - 5.52
by taking e^x for both side of the equation we can get [A]
∴[A] = 0.0012 mol dm^-3