purpose of adding CaCO3

In such titration known asMohr method, sodium chloride solution is put into a conical flask and a fewdrops of potassium chromate(VI) solution is used as an indicator. The sodium chloride solution is titratedagainst silver nitrate solution which is added from a burette.

The solubility of silver chromate is higher than thatof silver chloride. So, silver chloride will be firstly formed.
Ag^+(aq) + Cl^-(aq) → AgCl(s)
Only when no chloride ions (actually extremely small amount of chloride ions) areleft, silver chromate precipitate will be formed. Therefore, when the end pointof the titration is reached, a permanent pale red coloration is observed due tothe formation of a very small amount of brown-red silver chromate(VI) precipitate.
2Ag^+(aq) + CrO4^2-(aq) → Ag2CrO4(s)

However, it is difficult to observe such pale red coloration. Moreover, thefact that silver chloride precipitate turns grey (or even turn purple) in lightmakes such observation even more difficult.

A simple solution to the problem is to add some calcium carbonate powder to thereaction mixture in order to enhance easier observation. This is because theamount of white powder is increased and such white powder will not change itscolour when put in light.