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Solubility and the pH of the solution | Chemistry | Khan Academy - YouTube
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- [Voiceover] Let's say we
have a saturated solution
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of calcium fluoride in equilibrium
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with solid calcium fluoride.
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So in our beaker, here's
some solid calcium fluoride,
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which is a slightly
soluble ionic compound.
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And, in solution, we have
a saturated solution,
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consisting of calcium two
plus and fluoride anions.
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Now let's say we add some acid.
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So we add a source of
protons to our beaker
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and we observe what happens.
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Well, we would see some of
our calcium fluoride dissolve
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and let's say we add enough acid
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to completely dissolve
our calcium fluoride.
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So what happened here?
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We added acid, right, we added H plus,
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therefore we decreased the pH
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and we saw our calcium fluoride dissolve.
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So we increased the solubility, right?
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We increased the solubility
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of our slightly soluble
compound calcium fluoride.
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So let's see if we can
figure out what happened.
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For adding H plus, we
know we have H2O present,
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so H plus and H2O give us H3O plus.
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So we have hydronium ions in solution.
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And we also have fluoride
ions in solution.
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And fluoride can function as a base.
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So the fluoride anions are
gonna function as a base
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and hydronium functions as an acid.
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If we have two fluoride anions,
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we would need two hydronium ions.
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So, hydronium acts as an
acid, it donates a proton
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and fluoride anion acts as
a base, it accepts a proton,
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so we make HF, we make 2HF here.
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And if H3O plus donates a proton,
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we also have two waters.
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So we would make 2H2O here as well.
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So let's think about what we're doing
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to the concentration of
fluoride ions in solution.
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The fluoride ions are reacting,
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so therefore we're
decreasing the concentration
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of fluoride ions in solution.
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And remember Le Chatelier's Principle,
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a system disturbed from equilibrium
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will shift its equilibrium to
relieve the applied stress.
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So here, the stress is
decreased concentration
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of one of our products.
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Our equilibrium shifts to
make more of our product.
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And, so therefore, more
calcium fluoride dissolves
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and we've increased the
solubility of calcium fluoride.
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We could ever write the overall reaction
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for what's going on here.
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So the net reaction,
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we can see that we have
two fluoride anions
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on the reactant side.
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We also have two fluoride
anions on the product side.
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So now we can just take these two
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and say that those are our reactants.
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So we have calcium fluoride
plus two hydronium,
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plus two H3O plus
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and we would get, for our products,
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we would get these guys over here.
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We would get Ca2 plus, so calcium ions.
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We would make 2HF, so a solution
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and also water.
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So this is another way to
think about what's happening,
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if you add acid to calcium fluoride,
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you're going to get
this for your products.
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So, adding protons, adding acid
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or decreasing the pH,
increased the solubility
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of this, slightly soluble, compound,
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but this isn't always true.
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It depends on what compound
you're talking about.
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For example, for silver chloride,
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this is another slightly soluble compound,
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but adding acid does not affect
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the solubility of silver chloride.
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Now let's think about why.
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If you add acid, you're
increasing the concentration
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of hydronium ions in solution
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and what happened above,
was the fluoride anions
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reacted with the hydronium
ions to shift the equilibrium.
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But here, the chloride anion
won't react with hydronium
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and that's because the chloride
anion is a much weaker base
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than the fluoride anion,
so let's think about that.
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Up here, the fluoride anion,
if you think about HF,
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the conjugate base is F minus.
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Down here, for the chloride anion,
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it be HCl, with the conjugate
base being Cl minus.
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The stronger the acid, the
weaker the conjugate base,
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so HCl is a strong acid, a
much stronger acid than HF
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and therefore Cl minus, the
conjugate base, is much weaker.
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So the chloride anion is a weaker base
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than the fluoride anion.
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So the chloride anion isn't going to react
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with hydronium and therefore
we're not decreasing
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the concentration of one of our products
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and so our equilibrium doesn't shift
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and the solubility of silver
chloride is unaffected
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by the addition of an acid.
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So decreasing the pH for silver chloride
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won't increase the solubility.
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So you need to think about,
if the base is strong enough
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to react with the acid that you're adding.
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