Which Sailor Moon episode is it when Mina gets her pure heart crystal taken?

The episode where Mina (Minako Aino, Sailor Venus) gets her Pure Heart Crystal taken is Episode 18: The Last Pure Heart.

In this episode, the Dark Kingdom's forces attack Mina and steal her Pure Heart Crystal, leaving her vulnerable and weakened. This forces her to temporarily relinquish her role as Sailor Venus, as she needs to recover her power.