Currently, the Scientist's flasks get thrown a set distance from the player, making it impossible to hit zombies that are actually chasing you.
To fix this, I have two suggestions:
Flasks explode when they come into direct contact with a zombie before hitting the ground. This would be the simplest solution, but would weaken the effect, as the flasks can only be thrown at the edges of a crowd of zombies, rather than directly in the middle.
Flasks land directly on your crosshair within a certain radius. If the crosshair's between the player and the current set distance flasks are being thrown, the flask lands in the location the crosshair was when the flask was thrown. If the crosshair is outside of that radius, it lands the current set distance away from the player.