You are the manager of a chemical stockroom, and find a bottle containing approximately one liter of a clear and colorless solution of unknown identity and concentration. Your only clue to its identity is that it was found between bottles of silver fluoride and sodium fluoride, so it is likely an aqueous solution of one of those two compounds. Develop and write out a precise procedure, which includes all glassware, reagents, and steps to determine the identity and concentration of the unknown solution. Write the calculations that are needed to determine the concentration of the solution.

 

Going by the clues that it is between Silver Flouride (AgF) and Sodium Fluoride (NaF) and since it is an aqueous solution , the 1 liter bottle is likely to be Sodium Chloride( NaCl).

Going by the reaction,

AgF + NaCl → AgCl + NaF

Here, the color of AgCl is white, hence the solution cannot be AgCl.

Determination of NaCl

Determination of NaCl can be done by Mohr's Method or Volhard's method. But results in Volhard's method are more accurate. Its uses the method of back titration with Potassium Thiocynate which forms a AgCl precipitate . Prior to titration, excess AgNO₃ is added to the NaCl solution so that all the Cl^- ions react with Ag⁺.

Note that the problem also has a clue that excess reagents are present in the lab.

Fe³⁺ is then added as an indicator and the solution is titrated with KSCN to form a silver thiocyannite precipitate (AgSCN). Once all the silver has reacted, a slight excess of SCN^- reacts with Fe³⁺ to form Fe(SCN)₃ dark red complex.

The concentration of Cl- is determined by subtracting the titer findings of Ag⁺ ions that reacted to form AgSCN from the AgNO₃ moles added to the solution. This is used because pH of the solution is acidic. If the pH of solution is basic, Mohr's method is used.

Reactions

Ag⁺_(aq) + Cl^-_(aq) → AgCl_(aq)

Ag⁺_(aq) + SCN^-_(aq) → AgSCN_(aq)

Fe³⁺_(aq) + SCN^-_(aq) → [FeSCN]^2-_(aq)

Equipment Needed

1. boiling chips
2. 500 mL volumetric flask
3. 10 mL and 100 mL measuring cylinders
4. conical flasks
5. Bunsen burner, tripod and gauze
6. burette and stand
7. 50 mL pipette

Solutions Needed

1. Concentrated nitric acid (6 mol/L)
2. Silver nitrate solution: (0.1 mol/L). If possible, dry 5 g of AgNO₃ for 2 hours at 100°C and allow to cool. Accurately weigh about 4.25 g of solid AgNO₃ and dissolve it in 250 mL of distilled water in a conical flask. Store the solution in a brown bottle.
3. Potassium thiocyanate solution: (0.1 mol/L). Weigh 2.43 g of solid KSCN and dissolve it in 250 mL of distilled water in a volumetric flask.
4. Potassium permanganate solution: (5%) Add 1.5 g KMnO₄ to 30 mL of distilled water.
5. Ferric ammonium sulfate solution: (saturated) Add 8g of NH₄Fe(SO₄)₂.12H2O to 20 mL of distilled water and add a few drops of concentrated nitric acid.

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Titration

1. Use a volumetric cylinder to measure 100 mL of the sample solution (be as precise as possible) and pour it into a conical flask.

2. Add 1 mL of saturated ferric ammonium sulfate solution as indicator.

3. Titrate the unreacted silver ions with the 0.1 mol/L potassium thiocyanate solution. The end point is the first appearance of a dark red colour due to the ferric thiocyanate complex.

4. Repeat the titration with 100 mL samples of the solution until you obtain concordant results (titres agreeing with 0.1mL)

Result Calculations

1. Determine the average volume of potassium thiocyanate used from your concordant titres.

2. Calculate the moles of potassium thiocyanate used.

3. Use the equation of the reaction between silver ions and thiocyanate ions

Ag⁺_(aq) + SCN^-_(aq) → AgSCN_(s)

to calculate the moles of unreacted silver nitrate in 100 mL of sample, and multiply the figure by five to determine the total moles of unreacted silver nitrate (the excess) in the 500 mL volumetric flask.

4. Calculate the moles of silver nitrate in the 50 mL of solution that was added during the sample preparation.

5. Calculate the total moles of silver nitrate that reacted with the salt from the cheese by subtracting the moles of unreacted silver nitrate (the excess) from the total moles of silver nitrate added to the sample.

6. Use the equation of the reaction between the silver ions and the chloride ions to calculate the moles of sodium chloride in the sample.

Ag⁺_(aq) + Cl^-_(aq) → AgCl_(s)

7. Calculate the concentration of sodium chloride in the sample as grams of salt per 100 g sample (% salt).