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Current time:0:00Total duration:11:02

Video transcript

I've drawn a bunch of titration curves here so let's see if we can review everything we've learned to kind of have a more holistic understanding of interpreting these things so the first thing to look at is which of these are the titration of acids versus bases and everything I've done now is acids but the the logic for base titration is the exact same thing as acid so for example these are acid titrations we start with low PHS and all of these the this axis is pH I should have drawn that ahead of time before I asked you the question but I think you knew that already so before we add any of the titrant or the reagent in this reaction we're starting with a low pH so this is kind of our starting point so we have a low pH there we have a low pH there so these are both clearly acids here we're start our starting point before we start titrating at all it's a high pH or high pH so both of these are bases let me write that down so these are both bases I'll just do a base these are clearly both bases base titration this is an acid titration acid acid now we haven't covered bases but it's the same exact idea in an acid titration you start with an acid and you add a strong base to it to stop up all of the acid until all of the acid is sopped up and you hit the equivalence point you hit the point that all of the acid is sopped up and now as you add more and more strong base you're making it super basic so in this acid our equivalence point is over here and in this acid our equivalence point is over here this is how much solution we had to add to stop up all of the acid right there so given what we already know which one's a strong acid which one's a weak acid well this one when we sopped up all of the acid we have a completely neutral solution so this must have been a strong acid there's nothing left everything has kind of been converted to water in its natural state pH of seven and you know we might have had some neutral leftover conjugate bases there but since it was a strong acid those conjugate bases don't do anything there they don't add anything to the pH they're not really basic the hydrochloric the chlorine and hydrogen chloride the chlorine Aion isn't really doesn't in change the pH so this is a strong acid and this one when we got to the equivalence point when we had used up all of the acid in the solution and then we hit this inflection point where any o H we added was significantly increasing the pH R when we hit that equivalence point our pH was already basic and that's because we had all of the conjugate base of the weak acid which does make the solution more basic so this is a weak acid and in both of these situations we were increasing the concentration of O H - maybe by adding sodium hydroxide to the to the solution a strong base now in these situations we start with the base and we add a strong acid to it maybe we're starting in whatever base we're adding hydrogen chloride something that will stop up the O H here we want to stop up the o H and bring its concentration down until some point that we've chopped up all the O H all of the base is gone or for most of it it's gone and we're in this situation we're in a completely neutral situation so when we stopped up all the base we're completely neutral no basic conjugate bases left so this is a strong base strong base and here the titration we're increasing the hydrogen the solution or the hydrogen concentration to stop up all of the base same thing here we're sopping up all of the base we start over here but over here the inflection point happens right over here so we've stopped up all of its base but some of its conjugate acid is still left over even after we've chopped up all of its base so we end up with a slightly negative pH at the equivalence point so this is a weak base and let me actually draw that reaction for you just cuz that might be a little kind of remember a basic a weak base looks something like this maybe it's a - is in equilibrium at a second equilibrium arrows a little too wild for my blood is equilibrium with with a H it grabs hydrogen ions from the surrounding water everything is in an aqueous solution so if you add say hydrochloric acid to this remember Hydra HCl disassociates completely into hydrogen ions plus chlorine and ions if you add hydrochloric acid to this these things are going to just completely stop up these things so we keep stopping up those things are our concentration of O H goes down and down and down until some point that and as we stop up this our base our reaction goes in that direction because Lucia Talia's principle more and more of this is going to get formed into this and that until some point we're out of that out of that and we have a ton of this left and so our equivalent point was when we're out of this stuff and we were adding more hydrogen's we're getting really acidic really fast but we have a lot of the conjugate acid there in the solution already so we're gonna have a acidic equivalence point now let me give you an actual problem just to hit all the whole points home because everything I've done now has been very hand wavy and no numbers so let me draw one let me draw a weak acid and you'll recognize it because you're good at this now but I'll deal with some real numbers here so let's say that's pH of seven we're gonna titrate it it starts off at a low pH because it's a weak acid and as we titrate it its pH goes up and then it hits the equivalence point and it goes like that the equivalence point is right over here and let's say our reagent that we were adding is sodium hydroxide sodium hydroxide and let's say it's a point two molar solution and this is the point that we added I'm gonna use some I've been using to round numbers so I'll use a 700 milliliters of sodium hydroxide is our equivalence point right there so the first question is how much of our weak acid did we have so what was our original concentration of our weak acid this is just a general placeholder for the acid so original concentration of our weak acid well we must have added enough moles of O H at the equivalent point to cancel out all of the moles of the weak acid and whatever it's hydrogen it was out there but the main concentration was from the weak acid so this must have had this 700 milliliters of our reagent must have the same number of moles as the number of moles of weak acid we started off with and let's say our solution at the beginning was 1 listen maybe this is 3 liters 3 liters to begin with before we started titrating and obviously as we add as we add reagent we're adding some volume to the solution but let's just say that in the beginning we were started with 3 liters so how many moles have we stopped up well how many moles of O H are there and 700 milliliters of our solution well we know that the concentration of our solution we know that we have what we have point 2 moles per liter of Oh H and then we know that we don't have x 0.7 liters right we don't 700 milliliters is 0.7 liters so how many moles have we added to this situation see 2 times 7 is 14 and we have two numbers behind the decimal so it's point 1 4 so 700 milliliters of point 2 molar sodium hydroxide point 2 molar sodium hydroxide and we have 700 milliliters of it or 0.7 liters we're going to have point 1 4 moles of essentially Oh H that we put into the solution which means that it canceled out completely with point what the same number of moles of our original of our original of our original acid so that means that the original concentration of our acid is equal to 0.14 moles that's how many moles we had and we know that our original solution before we start titrating at all is 3 liters remember you have to these the molecules are cancelling directly with each other so that's why I wanted to figure out how many actual atoms or molecules of Oh H did I add those canceled out with the exact same number of atoms of our weak acid and so this is how many atoms of our molecules of our weak acid we must have star it off with and so you divide that by the number of leaders and then you have your original molarity so 0.14 divided by 3 divided by 3.0 4 6 so 0.046 so your initial your initial concentration of your acid that you're where their mystery acid was 0.046 molar fair enough now the other question is is what is the what is the KA of our our the pKa of our mystery acid pKa of our mystery acid well we just go to the half equivalence point so we said okay what was the ph of our of our titration curve or of our solution when we're at the half equivalence point so when we had only added 350 milliliters of our of our reagent of our strong base to the solution where did you go there and you say ok the pH was maybe this number is 5 pH is equal to 5 and we know from the last video that if you take this half equivalence point the pH is equal to the pKa the negative log of our equilibrium constant so there we figured out the equilibrium constant as well it's equal to 5 so all of this this all of this this titration curve and all of this I'm just showing you how experimentally you can you take some mystery acid or base you add strong acid or base to it you plot out this curve and then you can kind of pinpoint some of the properties the concentration of your original acid or base and if and this is only if you're dealing with a weak acid or base you can figure out its equilibrium constant obviously if you take a strong acid you say oh my half equivalence point is is here and so therefore this must be the the equilibria the pKa of there's no there is no equilibrium constant for a strong acid and there is no equilibrium constant for a strong base because they're not an equilibrium they disassociate they disassociate completely anyway hopefully you have a good understanding of titration now
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