Why you cannot add 0.3 moles of calcium fluoride directly to 1L of water to make a 0.3 M solution ?
need help ASAP!

Because it's not soluble in water, need to be heated acidic solution

Explanation:

Calcium Fluoride Formula: CaF2  Comprises of Ca2+ and F−

H2O = Water H+ O2- OH-

Insoluble in water

In order to dissolve a salt, you have to break apart the ions and hydrate them via a solvent.

Need to read

HSAB concept( Pearson acid-base concept) is an initialism for "hard and soft (Lewis) acids and bases"

So F- is a Halogen which is a Hard Base

And OH- is a Hard base as well

And H- is a soft base

So soft acids react best with soft bases and hard acids react best with hard bases.

Bases don't react with other bases.