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Current time:0:00Total duration:5:11

Solubility and the pH of the solution

Video transcript

let's say we have a saturated solution of calcium fluoride in equilibrium with solid calcium fluoride so in our beak here here's some solid calcium fluoride which is a slightly soluble ionic compound and in solution right we have a saturated solution consisting of calcium 2 plus and fluoride anions now let's say we add some acid right so we add a source of protons to our beaker and we observe what happens well we would see some of our calcium fluoride at dissolve and let's say we add enough acid to completely dissolve our calcium fluoride so what happened here we added acid right we added H+ therefore we decreased the pH and we saw our calcium fluoride dissolve so we increased the solubility right we increase the solubility of our slightly soluble compound calcium fluoride so let's see if we can figure out what happened all right for adding H+ right we know we have h2o presence so H+ and h2o give us h3o plus so we have hydronium ions in solution and we also have fluoride ions in solution and fluoride can function as a base all right so the fluoride anions are going to function as a base and hydronium functions as an acid if we have two fluoride anions we would need two hydronium ions so hydronium acts as an acid a donates a proton and the fluoride anion acts as a base it accepts a proton so we make HF right we make two HF here and if H three o plus donates a proton we would also have two waters right so we would make two h2o here as well so let's think about what we're doing to the concentration of fluoride ions in solution the fluoride ions are reacting so therefore we're decreasing the concentration of fluoride ions in solution and remember the Shapley A's principle a system disturbed from equilibrium will shift its equilibrium to relieve the applied stress so he here the stress is decreased concentration of one of our products our equilibrium shifts to make more of our products and so therefore more calcium fluoride dissolves and we've increased the solubility of calcium fluoride we could even write the overall reaction for what's going on here so the net reaction alright we can see that we have two fluorides on two fluoride anions on the on the reactant side we also have two fluoride anions on the product side so now we could just take these two and say those are our reactants so we have calcium fluoride alright plus two hydronium plus two H three o plus and we would get we would get for our products we get these guys over here we would get ca 2 plus so calcium ions we would make to HF alright so a solution and also water alright so this is another way to think about what's happening if you add acid to calcium fluoride you're going to get this for your products so adding adding protons adding acid or decreasing the pH increase the solubility of this slightly soluble compound but this isn't always true it depends on what compound you're talking about for example for silver chloride right this is another slightly soluble compound but adding acid does not affect the solubility of silver chloride and let's think about why if you add acid you're increasing the concentration of hydronium ions in solution and what happened above was the fluoride anions reacted with the hydronium ions to shift the equilibrium right but here here the chloride anion won't react with hydronium that's because the chloride anion is a much weaker base in the fluoride anion so let's think about that up here the fluoride anion if you think about HF right the conjugate base is f- down here for the chloride anion it be HCl with the conjugate base being CL minus the stronger the acid the weaker the conjugate base so HCl is a strong acid right a much stronger acid than H F and therefore CL minus the conjugate base is much weaker so the chloride anion is a weaker base in the fluoride anion and so the chloride anion isn't going to react with hydronium and therefore we're not decreasing the concentration of one of our products and so our equilibrium doesn't shift and the solubility of silver chloride is unaffected by the addition of an acid so decreasing the pH for silver chloride won't increase the solubility so you need to think about you need to think about if the base is strong enough to react with the acid that you're adding