The impossible event has probability 0. The complement of an event A is everything that is not A, with probability P(A') = 1 - P(A). For the impossible event (P=0), its complement is the certain event.

Probability of complement: $P\left(A^{\prime }\right)=1-P\left(A\right)$

For impossible event A: $P\left(A\right)=0$

So $P\left(A^{\prime }\right)=1-0=1$

Final answer: 1