I've been asked to state the function of PbCO3 in this reaction

Question

In the separation of Ru, Ir, Os, Rh and Ag (now AgCl, after dissolution in Aqua regia), how does addition of PbCO3 and HNO3, bring out AgNO3

Answer 1

2 AgCl  +  Pb(+2)  +  2 NO3(-) ------------>  2 AgNO3  +  PbCl2 (↓)

The lead provides a cation for the chlorine anions, which then precipitates.  The precipitate holds the Cl out of solution so the reactants can push further to the right.

Answer 2

The reaction George C. wrote is fine; but I would hazard a guess that the Pb(2+) is taking the Cl(1-) out of solution based more on a mass-action effect than by essential solubility, since K(sp)'s for AgCl and for PbCl2 are respectively about 1.8 * 10^-10 and 1.7 * 10^-5. That's quite a whopping difference, especially considering that one is a square and the other a cube for exponent, and NOT in favor of PbCl2 over AgCl precipitation!