Introduction
The capability of substances to react is directly related to their solubility. The reactions with insoluble salts are performed only on their surface. Therefore, the formation of new substances is limited. The living organisms are able to absorb the compound only from water solution, i.e. most of the insoluble compounds are inert for living organisms. The industrial-scale reactions are also performed in solutions (Myers 130). Hence, understanding the solubility and its characteristics is important for the qualified specialist.
During the lab, the reactions with different substances are realized. The qualitative characteristic of the solubility, the solubility product constant (Awonusi 399) is determined experimentally for silver acetate. The solubility product is calculated basing on the concentrations of ions in the saturated solution of AgCH3COO:
Ksp = [Ag+] [CH3COO-].
The solubility product is evaluated using the concentration of silver ions in the solution (Vining 594).
Materials and Methods
A. 10 ml of the lead chloride over-saturated solution had been prepared. The excess salt had been filtered, and HCl (3M) was added.
B. 5 ml of 0.1 M solution of Pb(NO3)2 had been added to:
- 5 ml of Na2C2O4
- 3 ml of 3M H2SO4.
After formation of the precipitate, 5 ml of HNO3 (3M) had been added.
C1. 10 ml of 10% Na2SO4 and a drop of 10% BaCl2 had been mixed.
C2. 10 ml of 10% Na2SO4 and some drops of CaCl2 (30%) had been mixed.
C3. 10 ml of 10% Na2SO4 and 8 drops of 50% H2SO4 had been swirled. To the same test tube, CaCl2 (30%) had been added by drops and mixed.
D. The solutions of 0.2M, 40 ml AgNO3 and 0.3M, 30 ml CH3COONa had been mixed. The excess precipitate had been removed by filtration. The clear solution had been analyzed to determine the concentration of Ag+ ions by titration with 0.02M potassium thiocyanide (Hummel and Zumdahl 311)
Results
A. The prepared solution is of lead chloride is cloudy. When filtration had been performed, the solution had gone clear. The salt remained on the filter.
B. The reactions:
Pb(NO3)2 + Na2C2O4 → PbC2O4↓ + 2 NaNO3
Pb(NO3)2 + H2SO4 → PbSO4↓ + 2HNO3
In both test tubes, the liquid was cloudy due to formation of the solid particles. After some time, the solid salts (PbC2O4 and PbSO4) had precipitated and the solutions had changed to clear.
The addition of the nitric acid had resulted in formation of the soluble salt of lead Pb(NO3)2:
PbC2O4 + 2HNO3 → Pb(NO3)2 + H2C2O4
PbSO4↓+ 2HNO3 → Pb(NO3)2 + H2SO4
The solutions had been clear.
C1. The reaction:
Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl
As the drop of barium chloride touched the solution, the formation of the small particles; after mixing, the solution had become cloudy.
C2. The reaction:
Na2SO4 + CaCl2 → CaSO4↓ + 2NaCl
Although the reaction is similar to the previous experiment, the observations are different. The first drop of calcium chloride had not formed the precipitate. As more substance had been added, the flakes of the precipitate appeared.
C3. When Na2SO4 and H2SO4 had been mixed, the reaction had not been observed, only the solution had got slightly yellow. The addition of CaCl2 launces the reaction:
Na2SO4 + CaCl2 → CaSO4↓ + 2NaCl
D. When the solutions had been mixed, the following reaction occurred:
AgNO3 + CH3COONa → NaNO3 + CH3COOAg ↓
The solution had been filtered to remove the precipitate, and the aliquot had been taken to determine the concentration of silver ions.
The titration results and calculations of silver ions ([Ag+]=VCVal):
Trial 1: 14.4 ml. [Ag+]1=14.4ml∙0.02M5ml=0.0576 M
Trial 2: 14.9 ml; [Ag+]2=14.9ml∙0.02M5ml=0.0596 M
Trial 3: 16.5 ml; Ag+3=16.5ml∙0.02M5ml=0.0660 M
Basing on the stoichiometry of the reaction between AgNO3 and CH3COONa and their quantities in the solution, [CH3COO-]1 = 0.072 M, [CH3COO-]2 = 0.074 M, [CH3COO-]3 = 0.0804 M. Hence, the solubility product:
Trial 1: Ksp1 = 0.0576·0.072 = 4.1·10-3
Trial 2: Ksp2 = 0.0596·0.074 = 4.4·10-3
Trial 3: Ksp3 = 0.066·0.0804 = 5.3·10-3
Discussion
A. The solution is cloudy because of the salt particles that had not dissolved. The salt remained on the filter because the quantity of the salt added to the solution is more that the solubility of the salt.
B. The precipitates formed in the reaction; after addition of the nitric acid, they dissolved since all nitrate salts are soluble in water.
C1. The formation of the small particles indicates precipitate formation, and the solution becomes cloudy when the precipitate is well mixed and the solid particles are evenly distributed in the volume.
C2. The late precipitate formation is explained by higher solubility of calcium sulphate, and more salt has to be added to form more calcium sulphate, which then precipitates.
C3. In this case, the precipitate of calcium sulphate had been formed, as the first drops had been introduced to the solution. The quick formation of the precipitate is explained by the common ion effect (Vining 603), the result of sulphuric acid presence.
D. The mean value of solubility is 4.6 ·10-3 ± 33%. The high value of the confidence interval appeared due to errors at experimental measurements (Natrella 23-2), namely the trial 3, which significantly differs from the other two values. The experiment could have been improved if one more trial was performed, and the trial 3 value had been removed as outlier (Natrella 17-4).
Conclusions
The experiments on solubility and precipitates formation had been performed on the example of lead, barium and calcium salts. The solubility constant for silver acetate had been calculated basing on the experimental data. The value was obtained and its accuracy assessed.
Works Cited
Awonusi, Ayorinde. Basic Principles of Calculations in Chemistry. Alief, Texas: Doves Brights, 2010. Print.
Hummel, Thomas J, and Steven S. Zumdahl. Student Solutions Manual to Accompany Chemical Principles, Seventh Edition, Steven S. Zumdahl. Belmont, CA: Brooks/Cole, Cengage Learning, 2013. Print.
Myers, Richard L. The Basics of Chemistry. Westport, Conn: Greenwood Press, 2003. Internet resource.
Natrella, Mary G. Experimental Statistics. Dover Publications, 2013. Internet resource.
Vining, William J. General Chemistry. , 2014. Print.
