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pH, pOH of strong acids and bases

Video transcript

we know that if we leave water to its own devices so you have some h2o that it's an equilibrium with the auto ionized version of itself so a little bit of it will turn into some hydrogen ions and we know that this really takes a form of hydronium that these attach themselves to other water molecules and it could be h3o but we'll write it write it as a hydrogen ion right there which is really just a free-floating proton plus some hydroxide ion and we also know that in kind of an equilibrium state at 25 degrees Celsius at 25 degrees Celsius and remember equilibrium constants and equilibrium reactions only dependent on the temperature nothing else you don't well for a given for given molecule of course so 25 degrees Celsius we also know and we did this two videos ago that the equilibrium constant and we could just as a review that's the concentration of the products divided by the concentration of the reactants but the reactants in this case is just water it's the actual solvent and if the the reactant is what you're it's everywhere so if you just go back to that intuition example the probability of finding it is one in any so it's it's just always there so you don't include it so you could just say it divided by one or whatever and this is equal to the equilibrium constant of water we learn that that's ten to the minus fourteen because water by itself will have a hydrogen concentration of 10 to the minus 7 and a hydroxide concentration of 10 to the minus seven and if you take a log of everything so if you take the pkew pkew what was that well if you put a P in front of something that means using the negative log of it so the negative log of 10 to the minus 14 the log base 10 of 10 to the minus 14 is minus 14 so the negative log is just 14 so P kW is 14 and that is equal to that is equal to well what's if I take the negative log of this side right here let me do that this is just a logarithm property this is more our math than chemistry so the log of h plus x times our hydroxide ion that's the same thing just logarithm properties is the same thing as minus log of h plus minus or you could say plus the minus log of o h minus and what is this well this is this is just the ph which is equal to the minus log this is ten to the minus seven right ten to the minus seven mile all of that is minus seven you have the minus in front so this pH is equal to seven and what is this this over here this is our Poh the minus log of the hydroxide concentration and of course that was also 10 to the minus seven and so our Poh is equal to the log of that is minus seven you have a minus in front it's equal to seven so you get there right you get right there that little formula that the pkew or the negative log of the equilibrium constant of water P kW is equal to the pH of water plus the Poh of water and this at 25 degrees Celsius this is the thing that's going to stay constant because we're going to start messing with these things by throwing acid and base into the water this thing is always going to be 14 at 25 degrees Celsius remember if you as long as you keep temperature constant and you're not messing too much with the molecule itself your equilibrium constant stays constant that's why it's called a constant so with all of that out of the way let's think about what happens if I if I throw some acid into a let's say I have some hydrochloric acid H let me I'll use use colors more creatively so some hydrochloric acid it's an in n aqueous solution we know that it disassociates completely we know that disassociates completely which means that we're just left with let me not do it like that we're left with the hydrogen ion which of course really attaches itself to another water molecule becomes hydronium plus the chlorine anion or negative ion right there and let's say that I start that I take let's say that my let's say I do this with one mole one molar one one molar or you know this also sometimes written as one capital M of hydrochloric acid so essentially what am i what am i doing I am taking one molar of hydrochloric acid literally means that I am taking one mole of HCl per liter of our whole solution which is mainly water it's an aqueous solution per liter of water right so how much what's my concentration going to be of these things right here or in particular what's the concentration of the H going to be well if this disassociated completely right so all of this stuff there's this is not an equilibrium reaction remember I only do a one ray arrow to the right there's no even small leftwards our this is a strong acid hydrochloric acid so if you really put one molar of this in the in an aqueous solution you're not going to see any of this you're going to just see this so you're going to have so the hydrogen concentration here the hydrogen concentration here in the aqueous solution is going to be equal to one molar and you know there's also going to be one molar of chloride anions but we don't care about that we want to figure out well we if I haven't said it already it would be nice to figure out what the pH of this solution is now that that thrown hydrochloric acid in it well the pH is just a hydrogen concentration hydrogen concentration so well it's the line they we already have the hydrogen concentration that's one molar or one mole per liter of solution so the pH the pH is going to be equal to the minus log base 10 of our hydrogen concentration of 1 which is equal 10 to the what power is equal to 1 well tend to the well anything to the zero power is equal to one including 10 so this is equal to zero minus zero is just zero so your pH is zero so if you have one molar of hydrochloric acid you have one molar of hydrochloric acid and you throw it into a an aqueous solution and I mean well I guess I'm saying it you're putting it into a solution when I tell you it's one molar so if you would take if you have a concentration of one mole per liter of solution where the solution is where the solvent is water you will end up with a pH of zero a pH of zero what was waters neutral so pH of so pH of water just without any acid in it that was equal to seven and this is considered a neutral pH now we know that if you were to have a solution an aqueous solution with one molar of hydrochloric acid we can say I'll do it in red because it's pH of HCL and water is equal to zero so obviously a low pH is more acidic and we went over that in previous videos and let's figure out what the Poh of hydrochloric acid is P o H of hydrochloric acid in an aqueous solution well we can this all goes back to Lycia talia's principle right if you go back to what we said before when we throw the hot when we essentially this is just pure water right here if we may put one molar of hydrochloric acid in here we're essentially just throwing a ton we're just throwing a ton of of hydrogen protons in there we're substantially increasing the concentration of this and elosha talia's principle says oh well that means that a lot of this is going be consumed and the reaction will go in this direction the equilibrium reaction will go in that direction but where remember water by itself only had a ten to the minus seven concentration we're throwing in a million a million of I mean that's one it was it was one ten millionth of a mole of a of a mole per liter now we're dry throwing in one what is that we're throwing in ten to the seventh we're throwing in ten million times as much hydrogen ions into that water so all of this stuff just gets consumed maybe it goes there and so this the concentration of this gets thrown down really far because we're throwing dumping so much and the concentration of this goes up because it can only consume so much of these guys there's only there's not that much of this there's only ten to the minus seventh molar of this so this ends up being one molar and if this has ends up being one molar because you know that ten to the minus seventh molar essentially you can kind of view it at all it gets consumed with this stuff over here what ends up being the concentration of the O H well we already know that the the PKW is 14 of water at 25 degrees and the pkew of water is equal to the pH of your solution plus your Poh so if your pH if your pH is 4 hydrochloric acid is 7 sorry is zero right we have one molar of hydrochloric acid then your Poh of one molar of hydrochloric acid is 14 so right here our Poh is equal to 14 now let's do the same thing with a base and figure out what its pH a strong base and I think you'll see that it's it's the opposite so let's say I had potassium potassium hydroxide the strong base so it completely disassociates in water two potassium cations positive charged ions plus hydroxide anions it completely dissociates so if I put and it's in an aqueous solution I should write that down aqueous aqueous solution aqueous solution just means we are in water of course and if we do if we essentially put 1 molar remember the concentration matters you can't just say Oh lauric acid has a pH 0 no you have to say one molar of hydrochloric acid has a pH of 0 and IQ actually I didn't write that let me write that 1 molar 1 molar and I'll leave you to figure out what the pH or the Poh of two molars of hydrochloric acid is or of 10 molar of hydrochloric acid and figure out what those PHS are but if we have if we have 1 molar so we have 1 molar of potassium hydroxide potassium hydroxide we have 1 molar of this what's and it completely disassociates when it's in water so you have none of this leftover then what's your what's going how much what's your concentration of O H well your O H concentration is going to be 1 molar right if you had 1 mole per liter of this you're going to have 1 mole per liter of this because all of this just disappears in the water so what is your Poh P o H is just the negative log of this the log of 1 is 0 the negative of 0 is 0 and then your pH your pH in this circumstance well you could say all of us a hydrogen concentration concentration is but you know that when you throw a bunch of this stuff it's going to stop up a bunch of hydrogen the hydrogen is going to go down a lot but you're like well how do I measure it well you remember at 25 degrees Celsius the equilibrium constant of water is equal to the pH plus the Poh we showed that at the beginning of the video so 14 is equal to your pH plus 0 that's our Poh in this case so our pH is 14 so if you have 1 molar of I use potassium hydroxide in this case but if you have 1 molar of a strong base let me write that down 1 molar 1 molar of strong base strong base remember strong is kind of an official term in chemistry it means complete disassociation you have a pH of 14 and you have a Poh of 0 if you have 1 molar of strong acid if someone says that they have something with a pH of zero that they would like to maybe throw at you you should you should you should decline because it will probably it'll probably hurt your chances of well anyway so let's say you have one molar strong acid that's a pH of 0 and a Poh Poh of 14 and anyway maybe in the next video I'll actually show you what that this might give you the impression this is an absolute scale that zero is acidic you can get and 14 is basically get when you get to pH but that's not that's not the case you can actually get above this or you can get below this this was just when you had one one molar of a strong acid if you had two molars of a strong acid or actually if you had 10 molars right let's say you get your hydrogen concentration - 10 - 10 molar so then what's your what's going to be your so if you had 10 molar or strong acid you apply that in an aqueous solution it is when I say its molar by definition what's your pH going to be your pH is going to be the minus log base 10 of 10 this is the log base 10 of 10 is 1 10 to the first power is 1 so this is equal to minus 1 so minus 1 pH would if you had 10 molar say hydrochloric acid or nitric acid or anything like that anyway that's all for this video I'll see you in the next one