Combining Cations and Anions

Introduction

After completing this experiment you should be familiar with the relative solubility of some common salts. Solutions of ten salts have been prepared. Five of the salts have a common cation, sodium (Na⁺). The other five salts have the a common anion, nitrate (NO₃^-). The number of drops of each salt should be kept constant, so that the only difference in mixtures is the cation from the first group and the anion from the second group.

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1. Add 2 drops of silver nitrate solution to the first 5 wells in the "A" row of a 96-well plate (A1, A2, A3, A4, and A5). (If a plastic sheet is used instead of the 96-well plate, make puddles of 2 drops each on the sheet.) Add 2 drops of bismuth nitrate solution to the first 5 wells in the "B" row of a 96-well plate (B1, B2, B3, B4, and B5). Repeat for all of the remaining nitrate solutions, assigning each metal ion to a row of its own.
   1. 0.1 M silver nitrate (AgNO₃)
   2. 0.1 M bismuth nitrate (Bi(NO₃)₃)
   3. 0.1 M zinc nitrate (Zn(NO₃)₂)
   4. 0.1 M copper (II) nitrate (Cu(NO₃)₂)
   5. 0.1 M calcium nitrate (Ca(NO₃)₂)
2. Add 2 drops of sodium chloride solution to each of the "1" wells in the first five rows: A1, B1, C1, D1, and E1. Add 2 drops of sodium hydroxide to each of the "2" wells in the first five rows: A2, B2, C2, D2, and E2. Repeat for all of the remaining sodium solutions, assigning each anion ion to a column of its own.
   1. 0.1 M sodium chloride (NaCl)
   2. 0.1 M sodium hydroxide (NaOH)
   3. 0.1 M sodium carbonate (Na₂CO₃)
   4. 0.1 M sodium sulfate (Na₂SO₄)
   5. 0.1 M sodium hydrogen phosphate (Na₂HPO₄)
3. Note and record observations for the 25 wells.
4. If the well contains a precipitate after adding the two solutions, fill in the corresponding square with the color of the precipitate and note any unusual observations.
5. If there is no evidence of a precipitate, do not write in the square. A clear, colorless solution, indicates that no precipitate is present.
6. Of the salts that were produced, which are soluble?
7. Write balanced chemical reactions for all combinations that produced a precipitate.

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Safety

Wear safety glasses at all times in the laboratory. Some of the chemicals are toxic. Do not ingest any chemicals. Silver nitrate solutions stain skin. Avoid contact with the silver nitrate solution. Wash all spills immediately with large amounts of water.

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Ag⁺ + Cl^- --> AgCl
Ag⁺ + OH^- --> AgOH
2 Ag⁺ + CO₃^2- --> Ag₂CO₃
3 Ag⁺ + HPO₄^2- --> Ag₃PO₄ + 3 H⁺

Bi³⁺ + 3 OH^- --> Bi(OH)₃ (same in both hydroxide and carbonate solutions)
Bi³⁺ + HPO₄^2- --> BiPO₄ + H⁺

Zn²⁺ + 2 OH^- --> Zn(OH)₂
Zn²⁺ + CO₃^2- --> ZnCO₃
3 Zn²⁺ + 2 HPO₄^2- --> Zn₃(PO₄)₂ + 2 H⁺

Ca²⁺ + 2 OH^- --> Ca(OH)₂
Ca²⁺ + CO₃^2- --> CaCO₃
3 Ca²⁺ + 2 HPO₄^2- --> Ca₃(PO₄)₂ + 2 H⁺

Cu²⁺ + 2 OH^- --> Cu(OH)₂
Cu²⁺ + CO₃^2- --> CuCO₃
3 Cu²⁺+ + 2 HPO₄^2- --> Cu₃(PO₄)₂ + 2 H⁺

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Collect waste Bi(NO₃)₃ and AgNO₃ in a "heavy metal container", and dispose of as required by local laws.

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• This list may be expanded to include any other salts, or substitutions can be made. It is suggested that the cations used are supplied by the nitrates, and the anions are supplied by the sodium salts.
• The Bi(OH)₃ precipitate may be missed. Sodium hydroxide must be added in amounts sufficient to neutralize the acid required in this solution. Also, the Bi(OH)₃ precipitate is gelatinous.
• Adding too much NaOH to a solution of Zn²⁺ may cause any zinc hydroxide formed to dissolve as zincate ion, Zn(OH)₄^2-
• This experiment can also be performed using puddles on acetate sheets. Laminating sheets of paper with matrices already printed on them, and with large, dark regions that permit easy observation of light-colored precipitates, works well. There is no base of experience yet about the number of weeks or months the plastic in the laminate holds up under student use.

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• Cations
  • 0.1 M Zn(NO₃)₂ (dissolve 3.0 g Zn(NO₃)₂•6H₂O in enough distilled water to make 100 mL of solution)
  • 0.1 M Cu(NO₃)₂ (dissolve 2.3 g Cu(NO₃)₂•2.5H₂O in enough distilled water to make 100 mL of solution)
  • 0.1 M AgNO₃ (dissolve 1.7 g AgNO₃ in enough distilled water to make 100 mL of solution)
  • 0.1 M Bi(NO₃)₃ (dissolve 15 mL of concentrated nitric acid in 50 mL of distilled water. Add 4.9 g Bi(NO₃)₃•5H₂O to the nitric acid solution. Add enough enough distilled water to make 100 mL of solution.)
  • 0.1 M Ca(NO₃)₂ (dissolve 2.4 g Ca(NO₃)₂•4H₂O in enough distilled water to make 100 mL of solution)
• Anions
  • 0.1 M sodium chloride (dissolve 0.59 g NaCl in enough distilled water to make 100 mL of solution)
  • 0.1 M sodium hydroxide (NaOH) (dissolve 0.40 g NaOH in enough distilled water to make 100 mL of solution)
  • 0.1 M sodium carbonate (Na₂CO₃) (dissolve 2.9 g Na₂CO₃•10H₂O in enough distilled water to make 100 mL of solution)
  • 0.1 M sodium sulfate (Na₂SO₄) (dissolve 3.2 g Na₂SO₄•10H₂O in enough distilled water to make 100 mL of solution)
  • 0.1 M sodium hydrogen phosphate (Na₂HPO₄) (dissolve 2.7 g Na₂HPO₄•7H₂O in enough distilled water to make 100 mL of solution)
• 96-well plate or acetate sheet
• toothpicks
• cotton swabs

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The Woodrow Wilson microscale version of this lesson was developed by:
Bob Trowell
Chester High School
Chester, SC

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