Answer to Question #102836 in General Chemistry for Beverlie

Question

1. You are given a white solid which could be either AgNO3 or KNO3. How would you determine the identity of the solid?

2. You are told that an acid is either hydrochloric acid or nitric acid. What simple test would you carry out to tell which acid it is?

3. You are given a solution and asked to determine which Group 1 cations are present in it. Addition of HCl results in the formation of precipitate. The precipitate is washed with hot deionized water. When the filtrate from hot water washing is tested with potassium chromate solution, a precipitate forms. When a solution of ammonia is added to the remaining precipitate from the addition of HCl, it completely dissolves. What do you conclude?

Answer 1

1. React with HCl.

AgCl not soluble in water.

2. React with AgNO3.

AgCl not soluble in water.

3. If a dilute solution of HCl is added to a solution containing ALL of the common metal ions, a white precipitate will be formed. This precipitate will contain AgCl(s), Hg₂Cl₂(s), and PbCl₂(s). If this solution is centrifuged to collect the solid material at the bottom of a test tube, the supernatant liquid can be removed (decanted). The remaining solid can be washed with distilled water so that the ONLY metal ions that remain are Ag⁺, Hg₂²⁺, and Pb²⁺. The reactions that occur are:

   Ag⁺(aq) + Cl^-(aq) -> AgCl(s)

   Hg₂²⁺(aq) + 2 Cl^-(aq) -> Hg₂Cl₂(s)

   Pb²⁺(aq) + 2 Cl^-(aq) -> PbCl₂(s)

The reactions are conducted in acidic solution, to avoid the possibility that these and other metal ions might be precipitated as hydroxides or oxides. This reaction of this group of ions with chloride ions allows them to be separated from all other metal ions.  Silver, mercurous, and plumbous ions have been designated as the Group I Cations.

   If addition of dilute HCl to a solution of metal cations results in a precipitate, this is conclusive evidence that at least one - and possibly all - of the Group I Cations were are present in the original solution. Additional tests are necessary to determine which of these three cations are present in the precipitate.