Remember that the concentration of the salt is not included in the solubility product constant equation, because it is a solid. Only the ion concentrations can affect the solubility of the salt in solution. When the solution is saturated, adding more salt will do nothing to the concentration of ions in the solution.

Scandium hydroxide dissociates according to this reaction.

As 0.1N HCl is added, it will quantitatively react with hydroxide anions to produce ScCl₃ and water.

Hydroxide will be removed from the above equilibrium, and the system will compensate by shifting the equilibrium to the right, according Le Chatelier's principle. As the equilibrium shifts to the right, more solid scandium hydroxide is hydrolyzed, resulting in increased solubility.

Since the system is originally at equilibrium and a stress is applied, Le Chatelier's principle is to be considered. By increasing the total pressure, the reaction will move in the direction toward which there are less molecules of gas.

Looking at the original reaction there are two moles of gas on the left and only one on the right, indicating that the reaction will shift to the right if the pressure is increased.

If lead nitrate is added to the reaction, it will dissociate into lead ions and nitrate ions. Because of the common ion effect, adding additional lead ions to the reaction will shift the reaction to the left and reduce the production of lead and chromate concentrations, keeping the solubility constant the same. (This is in accordance with Le Chatelier's principle.)

Adding sodium chromate increases the concentration of chromate ion in the solution, which shifts the reaction to the left due to the common ion effect. Thus, the solubility of lead chromate would decrease, as there would be an increased amount of solid lead chromate.  

Us the solubility constant to calculate the answer to this question. We know that:

and

Since  is a very small number, we can assume that the concentration of chromate, , will be very small compared to the concentration of the added lead, 0.019 M. We can use the two known values ( and lead concentration) to solve for the unknown chromate concentration:

We know that concentration, temperature, medium and catalyst all affect the direction and the rate of the reaction through the Le Chatelier's Principle, where a system tries to relieve stress when stress is being applied. In order to push the equilibrium towards the right, we need to increase volume, decrease temperature, increase concentration of A and B, and decrease the concentrations of C and D. In the choices provided, only roman numeral I provided a correction suggestion. By increasing temperature, the system would want to go to the side that does not produce heat (the left). By increasing the amount of D, it will tend to shift reaction away from the species being added. By decreasing the volume and thus increasing the pressure, the system would want to shift left towards the side with less moles of gas. 

The following will shift the equilibrium to the left:

• decreased volume (increased pressure)
• increased temperature
• decreased amounts of A and B
• increased amounts of D and C

Of the choices, all are correct except for Roman numeral I. Placing the vessel into an ice bath decreases the temperature of the equilibrium, and thus the system would want to shift towards the side that produces heat to replace the heat lost.