Introduction
The inorganic salts that dissolve in water form ions. Thus, the dissolved salts exist in the solution in ionic form. For example, lead chloride exists in the form of Pb2+ and Cl-; sodium sulphate in the form of Na2+ and SO42-. When the solutions of two salts are mixed, the reaction takes place only if precipitate, water or gas forms (Jespersen and Jespersen, 2010). In other case, the ions exist in the solution in equilibrium. The insoluble salts typically remain in the solution as the precipitate. However, even in case of the insoluble salts, there are still small concentration of the ions. Therefore, the term “insoluble” is relative, and all the insoluble substances have limited solubility. The solubility is characterized with the solubility product, which is the product of the ionic concentrations raised to the n-th power, where n is the quantity of the ions in the molecule. For instance, the solubility of lead sulphate is:
Ksp = [Pb2+] [SO42-].
The soluility products are tabulated values (Yalkowsky & He, 2003). When the value of the solubility is reached, the substance forms precipitate.
The lab assignment aims to test the formation of the precipitate and calculate the solubility product basing on the experimental values.
Materials and Methods
Precipitates Formation
The lead chloride was added to 10 ml of water. The solution was filtered to remove the precipitated salt, and 3M hydrochloric acid was added.
Two test tubes with 5 ml of 0.1 M lead nitrate were added with 5 ml of sodium oxalate and 3 ml of 3M sulphuric acid. After the precipitates had been formed, 5 ml of 3M nitric acid was added.
In the test tube, 10 ml of 10% sodium sulphate was mixed with 10% barium chloride, which was added dropwisely.
In the test tube, 10 ml of 10% sodium sulphate was mixed with 30% calcium chloride was mixed.
In the test tube, 10 ml of 10% sodium sulphate was mixed with 50% sulphuric acid (8 drops). Then, 30% calcium chloride was added dropwisely.
Solubility Product
The solutions of silver nitrate and sodium acetate (AgNO3: 0.2M, 40 ml; CH3COONa: 0.3M, 30 ml). The precipitate was filtered, and the solution was analyzed by titration with potassium thiocyanide (0.02M) in presence of Fe3+ ion.
Results and Discussion
Precipitates Formation
The solution of lead chloride is oversaturated, i.e. some lead chloride dissolved, and some formed precipitate. As a result, the solution is cloudy because of the dispersed particles of the precipitate. After filtration, the solution becomes clear, as the precipitate remains on the filter.
The reactions of lead nitrate with sodium oxalate and sulphuric acid:
Pb(NO3)2 + Na2C2O4 → PbC2O4 (precipitate) + 2 NaNO3
Pb(NO3)2 + H2SO4 → PbSO4(precipitate) + 2HNO3
The reactions take place, and the precipitate formed. Thus, the solution became milky since the small particles formed. The precipitated compounds sedimented, and the solution became clear.
When nitric acid was added, the precipitates dissolved and the soluble lead nitrate formed:
PbC2O4 (precipitate) + 2HNO3 → Pb(NO3)2 + H2C2O4
PbSO4 (precipitate) + 2HNO3 → Pb(NO3)2 + H2SO4
Since there were no solid substance in the solution, the solution in the test tube became clear.
The sodium sulphate and barium chloride react to form the barium sulphate:
Na2SO4 + BaCl2 → BaSO4 (precipitate) + 2NaCl
The barium chloride was added dropwisely, and as the drop was added, the white solid particles were seen. As the test tube was mixed, the solution looked cloudy.
The sodium sulphate and calcium chloride react to form the calcium sulphate:
Na2SO4 + CaCl2 → CaSO4 (precipitate) + 2NaCl
The addition of the first portions of calcium chloride did not show any changes. The next drops of calcium chloride resulted in formation of the solid particles, which was observed as cloudy solution. The “postponed” formation of the precipitate is explained by higher solubility product of calcium sulphate.
The mixture of sodium sulphate and sulphuric acid resulted in formation of the light yellow solution, no reaction took place. When the calcium chloride was added, the reaction took place (almost at once):
Na2SO4 + CaCl2 → CaSO4 (precipitate) + 2NaCl
The formation of the precipitate after introduction of the first drops is explained by the presence of the sulphate-ion, or common ion effect, which decreases the quantity of calcium necessary to form the precipitate.
Solubility Product
The mixture of silver nitrate and sodium oxalate results in the reaction:
AgNO3 + CH3COONa → CH3COOAg (precipitate) + NaNO3
The solution had been filtered to remove the precipitate, and the sample was used to analyze the solution for silver ions. The results of the calculations are related to the experiment are listed in Table 1. The solubility product is calculated:
Ksp = [Ag+] [CH3COO-].
The Calculations for the Experiment
The average solubility product is Ksp = (3.32 ± 0.1)·10-3.
Conclusions
The lab assignment tested the conditions of the precipitate formation in various conditions. The formation of the precipitates was observed. The common ion effect was studied and tested. The solubility product was calculated on the example of silver nitrate. The determinations were precise, and the difference in trials was caused by random experimental errors. The calculated value of the soluility product is (3.32 ± 0.1)·10-3.
References
Jespersen, N. D., & Jespersen, N. D. (2010). AP chemistry. Hauppauge, NY: Barron's Educational Series.
Yalkowsky, S. H., & He, Y. (2003). Handbook of aqueous solubility data. Boca Raton, Fla: CRC Press.
