a solid labortory unknown contains two substances, lead (II) carbonate and potassium carbonate. a 5.000 g sample of this unknown was treated with aqueous nitric acid to dissolve it, then, after futher treatment to adjust the acidity, it was treated with an excess of potassium chromate solution. the precipitate which formed, when dried carefully and weighed, has a mass of 2.697 grams. calculate the percent, by mass of (PbCO3) in this laboratory unknown. (hint: write a balanced equation, determine the formula of the ppt using the solubility chart, then grams of lead using mole ratio, then grams of PbCO3 using mole ratio).