Video Transcript
Which of the following can be employed as the precipitating agent in the gravimetric analysis of chloride salts? (A) HNO3 aqueous, (B) BaCl2 aqueous, (C) AgNO3 aqueous, (D) HCl aqueous.
Gravimetric analysis is a type of chemical analysis where the mass of a substance is determined accurately using an analytical balance. One type of gravimetric analysis is precipitation gravimetry. In this technique, the formation and mass of a precipitate is used to determine the mass of an analyte. A chemical reaction may occur when ions in separate solutions are mixed together. The combination of ions may produce an insoluble product. The insoluble product is known as a precipitate. The precipitate would need to be filtered, then washed with distilled water to remove any soluble impurities. Then, after thorough drying, the precipitate mass can be determined accurately.
If we’re to do gravimetric analysis of chloride salts, the chemical reaction that we use as the basis of the analysis must produce an insoluble chloride salt. The insoluble chloride salt will be the precipitate, and its mass can be determined accurately. Soluble chloride salts will release the chloride anion into the solution. This has the formula Cl− aq. The aq symbol means the ion is aqueous and it’s mobile in the solvent that it’s dissolved in. As a negatively charged ion, the chloride ion may react with positively charged ions or cations.
To enable the gravimetric analysis of chloride ions, we need to find a positive ion or cation that will react with the negative chloride ion and form an insoluble chloride precipitate. Finding an insoluble chloride salt is not as easy as it sounds as nearly all common chloride salts are highly soluble. Exceptions to this rule though are the chlorides of silver and some of the chlorides of lead and mercury. The insoluble chlorides of lead and mercury are not encountered very often, and they’re also very toxic compounds to deal with. If aqueous chloride ions encounter aqueous silver ions, solid silver chloride is produced as a precipitate.
To answer our question, we would need to find a precipitating agent that could yield aqueous silver ions. The only compound in our list that could yield the required aqueous cation is silver nitrate. This compound is much safer to use than mercury and lead compounds as a precipitating agent for chloride ions due to its fairly low toxicity. None of the other compounds in our list will yield cations that will give us a precipitate with chloride ions. Nitric acid will yield aqueous hydrogen ions. These will not form a precipitate with aqueous chloride ions. Barium chloride will yield aqueous barium two plus ions in solution. But as we can already see from our list, barium chloride is soluble. Hydrochloric acid also yields aqueous hydrogen ions. Hydrochloric acid doesn’t form a precipitate with aqueous chlorine ions either.
AgNO3 aqueous is the correct answer.
