[tulsahurricane1993]

This is what you are given. Start with .392 g of a stone, titrating that with .1 M HCl.
Ignore everything that is not Calcium Carbonate. Calcium Carbonate is 85.2% of that stone.
There will be two equivalence points. Now, find the pH at 1/2 first equivalence point?

So confused where to start with this, any help is greatly appreciated. 

[aHerraez]

Try writing the reactions, acid-base equilibria involved.
Then use the Henderson-Haselbalch equation for each equilibrium.