In the previous learning objective we learned that the common ion always decreases the solubility of an insoluble salt. In this learning objective the solubility of an insoluble salt. In this learning objective when and how pH affects the solubility of a salt. This is going to occur if one of the ions in the salt is an acid or a base. So here is an example. This will be a very simplistic example we will expand it to more complicated examples. Magnesium hydroxide is fairly insoluble. But it itself is a base and we see that we have that OH-. So the anion of this salt is a base. So if we look at that equilibrium and we just consider Le Chatelier's principle if we had the system at equilibrium and we just consider Le Chatelier's principle if we had the system at equilibrium and we when in and we added a base to that system would we say thinking about which way the equilibrium would shift that the solubility of the Magnesium Hydroxide would increase or the solubility would decrease? Well, if you said it would decrease you are correct. Why would it decrease? Well, if you add OH- anytime you add something on the right hand side it will shift to the left. To try and use it up. side it will shift to the left. To try and use it up. You are increase the solid that would be a representation of a decrease in solubility. So what is adding base, adding a base is the pH would be going up. As you add that base, then that would always that would always shift this salt to the left. So lets do the opposite. that would always shift this salt to the left. So lets do the opposite. Lets add an acid. Now when you add an acid, the H_3O+ concentration goes up. What does the OH- concentration do? Think about that and then answer the question. Well, if you said it would increase the solubility you would be correct. Why is that? Well, when the H_3O+ goes up by adding an acid always the OH- goes down because that product has to equal K_w. If that is going down then this is being removed. It is decreasing. And you are going to shift it to try and produce more. is being removed. It is decreasing. And you are going to shift it to try and produce more. SO in this case, as you decrease the OH- concentration by adding an acid or in other words as the pH goes down the solubility will increase. OK so lets consider another equilibrium. Here is an insoluble salt. For this substance it might quite be as obvious as to why the pH would affect the solubility of this salt. But F- is the conjugate base of a weak acid. Our weak acid would be HF and this is the conjugate base of HF. As a base it will be involved in its own equilibrium, so here it is F- in water, which is an aqueous solution and we end up with the F- having an OH in its equilibrium. Now this equilibrium here obviously going to be affected by pH because as we change the pH we are changing the amount of OH-. So lets walk our way through this consideration with some questions. So both of these equilibria are involved. We have got the lead fluoride, an insoluble salt dissolving. We have the F- of that salt undergoing ionization with water. We have the F- of that salt undergoing ionization with water. So if we add a base to this solution what affect will that have to this solution what affect will that have on equilibrium number 2? Well did you say it would shift it to the left? That is correct. If you are adding Well did you say it would shift it to the left? That is correct. If you are adding a base, in other words as the pH is going up it is going to shift that second equilibrium it is going to shift that second equilibrium to the left. And that would increase the amount of F- in solution. If you are increasing the amount of F- in solution how would the increase of the concentration of the F- affect the first equilibrium. Think about that and answer Well, did you say that would shift it to the left? You would be correct. So if we add Well, did you say that would shift it to the left? You would be correct. So if we add if the F- is going up, it shifts it to the left. You would be correct. So if we add if the F- is going up, it shifts it to the left. so what would we say the solubility of lead fluoride? When we add a base and the pH is going up? That would decrease the solubility. Anything you get more solid in there then you started with that is a decrease in solubility. Anything you get more solid in there then you started with that is a decrease in solubility. Now we looked at it in terms of adding base, we looked at it in terms of increasing the pH and we could go through these equilibria for adding an acid. If you add and acid, that decreases the OH- equilibria for adding an acid. If you add and acid, that decreases the OH- if you decrease the OH- that shift it to the right that decreases the F- concentration if you decrease the OH- that shift it to the right that decreases the F- concentration then we go to the first equilibrium. When that decreases the F- concentration then we go to the first equilibrium. When we go to the first equilibrium when the F- is decreasing that would shift it to the right. As it shifts it to the right increase the solubility. So you could go through and think about this for every salt that you come across, but there is a bottom line. The bottom line is this. If you look at a salt and the anion is a conjugate base of a weak acid which we have. Anion is a conjugate base of a weak acid. Then the solubility will always increase with decreasing pH. In other words, if you added and acid. So anytime to wanted to increase the solubility of a salt if you look at the anion and it is the conjugate base of a weak acid then you would be able to increase the solubility by adding acid. Bringing the pH down will increase the solubility of that insoluble salt. i want to show you in this demonstration how you can take an insoluble salt and redissolve it by adding an acid and lowering the pH. But first I want to create an insoluble salt. acid and lowering the pH. But first I want to create an insoluble salt. I got magnesium sulfate in this test tube and magnesium sulfate is very soluble. But I am going to add some ammonia. This is going to produce a precipitate of magnesium hydroxide. So I am going to produce the magnesium hydroxide. Now you can see that the test tube is getting cloudy Now you can see that the test tube is getting cloudy and that cloudiness is the formation of the magnesium hydroxide. Since the anion of magnesium hydroxide is the conjugate base of a weak acid Since the anion of magnesium hydroxide is the conjugate base of a weak acid we will be able to redissolve the magnesium hydroxide by adding an acid. I am adding hydrochloric acid here. magnesium hydroxide by adding an acid. I am adding hydrochloric acid here. As we lower the pH the magnesium hydroxide re-dissolves.
