When silver nitrate solution is first added to the chloride ion solution a white precipiate is formed. What is this precipitate?

The white precipitate is silver chloride.

The solution also contains potassium chromate(VI). Why isn't silver chromate(VI) precipitated at this point?

Silver chromate(VI) is not precipitated because its solubility product has not been exceeded. Silver chromate(VI) is more soluble than silver chloride.

What happens to the concentration of chloride ions in the solution as silver nitrate solution is added?

The concentration of chloride ions falls.

At the end point of the titration the chloride and silver ion concentrations are equal. If K_sp(AgCl) = [Ag⁺] [Cl^–] = 1.76 x 10^–10 mol² dm^–6, what will the silver ion concentration be at the end point?

The silver ion concentration will be the square root of K_sp.

So [Ag⁺] = √(1.76 x 10^–10) = 1.35 x 10^–5 mol dm^–3

The solubility product of silver chromate(VI) = 1.2 x 10^–12 mol² dm^–6. What concentration of chromate(VI) ions is required to exceed K_sp(Ag₂CrO₄) and therefore produce a red precipitate of Ag₂CrO₄ at the end point of the titration?

K _sp = [Ag⁺]² [CrO₄^2–] 

[CrO₄^2–] = K_sp / [Ag⁺]² = 1.2 x 10^–12 / [1.35 x 10^–5]² = 4.6 x 10^–3 mol dm^–3