Place the mixture into a container, add water, stir to dissolve all that will dissolve, filter, the ppt left in the paper will be Pb(CO3)2 while the filtrate will be aqueous Na2CO3. Evaporate the water to obtain the solid Na2CO3. One note: The Na2CO3 solid will contain traces of Pb from the Pb(CO3)2 since Pb(CO3)2 is slightly soluble in the water. The excess of Na2CO3 will reduce the solubility of Pb(CO3)2 but can't reduce it to zero; therefore, trace amounts of Pb^2+ will contaminate the Na2CO3.
