Common Ion Effect in Equilibrium

Dissolution and precipitation reactions are very important chemical reactions because it is applied to many aspects of the industries in medicine, food, water etc. The objectives of this laboratory experiment is to become familiar with dissolution and precipitation equilibria, develop a lab technique suitable for the determination of the solubility for a sparingly soluble salt, Ba(NO3)2 (s) at room temperature and measure the common ion effect in solubility of Ba(NO3)2 (s) in an acidic solution, HNO3 (aq).

Procedure: In this lab, the evaporation technique was used to determine the solubility of the salt at room temperature in water. First a 250ml beaker was weighed and the mass was recorded. Then 5. 0g of barium nitrate was weighed into the beaker. Next, 50ml of distilled water was added to the beaker with the barium nitrate sample. The sample was mixed for ten minutes using the magnetic stirrer. Afterwards, as much as possible of the barium nitrate solution was decanted without losing the solid barium nitrate.

To get rid of the excess water, the beaker placed onto a hot plate to evaporate the remaining water I the beaker. One the sample was dry, the mass of the beaker with the dry barium nitrate was weighed, and the mass of the barium nitrate dissolved was calculated. This procedure was repeated for the barium nitrate in acidic solution, HNO3(aq). All data was recorded and the mass of barium nitrated dissolved in acid was calculated.

Results/Discussion: From the evaporation technique, 4. 39g of barium nitrate dissolved in water and 2. 4g of barium nitrate dissolved in the acid. From that the concentration of barium nitrate in its saturated solution in water was 0. 336mol/L and 0. 179mol/L in HNO3. The concentration of Ba2+(aq) ions in saturated solution is the same as the concentration of the saturated barium nitrate solution by dissociation of barium nitrate ions into one mole of Ba2+ (aq) for every one mole of Ba(NO3)2(s).

The NO3- concentration however is double the concentration of barium nitrate due to the dissociation of 2 moles of NO3- for every one mole of Ba(NO3)2(s) plus the concentration of NO3- due to the dissociation of HNO3(aq). This came out to be 0. 672mol/L in water and 0. 858mol/L in nitric acid. The solubility of barium nitrate in water was calculated to be 87. 8g/L and 46. 8g/L in HNO3 acid. The molar solubility of the barium nitrate was by definition the same as the concentration of barium nitrate in its saturated solution which was 0. 336mol/L solubility in water and 0. 179mol/L for the solubility in acid.

The solubility product constant for barium nitrate in water was calculated using the experimental data to be 0. 52 in water and 0. 132 in HNO3 acid. From the results, it can be seen that the concentration of dissolved barium nitrate in water is more that the concentration of dissolved barium nitrate in nitric acid. And from that it is also seen that the solubility constant of barium nitrate in water is also more than the solubility constant of barium nitrate in nitric acid. These results prove that the common ion effect had decreased the solubility of barium nitrate.

The presence of NO3- ions in nitric acid and in barium nitrate caused the common ion effect because even before the barium nitrate dissolution in nitric acid, it already has the initial concentration of NO3- ions produced by nitric acid dissociation. The deviation of experimental values for solubility of barium nitrate in water and in nitric acid was 44. 7% and Ksp deviation was 13. 2%. This is correct because the solubility of barium nitrate in water is supposed to be greater than the solubility in nitric acid.

Conclusion: From the experiment, it can be seen that the solubility of barium nitrate in water is greater than the solubility of barium nitrate in nitric acid. This is due to the common ion, NO3-, in barium nitrate and nitric acid. The acid dissociation yielded a concentration of this ion already, so the dissociation of this ion from barium nitrate is an additional concentration of the ion. This is called the common ion effect. The experiment took the evaporation technique approach, but there are other ways to carry out this experiment to determine the solubility of barium nitrate.

One of such was is by using a specific amount of barium nitrate and dissolving it slowly in water until precipitation occurs. From that the amount that was dissolved will be known by taking the mass of the remaining amount of barium nitrate. This procedure was carried out as well during the lab experiment to test the experiment’s accuracy. From the results, it showed that more barium nitrate was dissolved using the evaporation technique. This is because in the alternate technique, it is hard to determine when the salt begins to precipitate; therefore it is not as accurate as the evaporation technique.