right so this video is going to be on uh so primarily bases and this term KW and I'm going to start with KW in a minute uh tiing into the bases because you need this kW to actually be able to calculate the pH of these bases um these notes I will put again in the description uh if you have a look you can uh just download this picture uh not very interactive but uh you can get all of that if you would like so best place to start with this I think is to talk about the idea of water and the fact that actually it does dissociate very slightly this ties into equilibrium um and what we've got is that water basic is an equilibrium of with its this is a simplified form of this you should really use the kind of whole hydroxonium thing but it's it's easy just to use it kind of like this now because it's an equilibrium we can write an equilibrium expression for this and there acids and bases is quite sort of intrinsically tied to uh the equilibrium topic because when it comes to weak acids you have these equilibrium occurring uh and these reversible reactions so we can basically what we can write is we can write uh an equilibrium for this um where yeah I'll leave that a second where we can do our usual thing H+ and O minus uh and we could do that over H2O and that would be an equilibrium for this an expression in uh in terms of KC for this equilibrium here of water dissociating into it h plus ions and O minus ions but we do something slightly uh different rather than using KC we use KW and we just don't include water at all so actually what we find is we have a sort of a new and I'll explain this in a second a new uh kind of expression I guess uh is set up this time in terms of thing called KW and KW is the ionic product of water okay so it's not KC it's KW and that is it right there KW equals H+ uh multiply by o minus again the concentrations of each which at this point I'm going to assume you understand what these square brackets mean so we cross out H2 and I'll start with that as opposed to starting with why we don't use K we cross out H2O we do that because the equilibrium lies really far to the left um and essentially H2O is almost kind of a constant really uh in itself and you don't really want constant and constant so what happens is rather than using KC we use a term called KW and that actually incorporates um sort of the water here and that's where we get this new term from so it's not just a kind of an any any odd uh new constant it's one that is incorporate in the water so actually we can remove from the bottom so the equilibrium is lying really far to the left very very small amounts of H+ and oh minus uh so actually but we can just say right we'll scrap that off we'll use KW rather than KC and everyone's happy you don't need to know about that if you can at this point your your brain has just melted it's coming out your ears it's coming out your nose it's possibly driving at your eyes then just use KW don't worry about any of this at all it's just kind of take into account sort of the idea I guess of uh some of the equilibrium stuff this did actually come up on one of the exam papers not going to say which one surprise um it it was asking about this but kind of using your common sense really in in some ways why would you not include water in the in the expression but there's your reasons anyway this is the important bit though if you like it's going to carry more of your marks uh more commonly okay um at I'll scroll this way slightly at uh room temperature so 298 kelv that's 25 degrees CSUS KW is has a value of 1 time 10 minus 14 and we can work out the units quite nicely mole Square DM minus 6 and that means you could do some various calculations to work out H+ and H minus as we're going to now where I'm going to say to you well what is the pH therefore of uh water at 298 Kelvin and at this point you might think well pH of water pH is the minus log of H+ ions so you need to find H+ ions but what's the O minus ions this is that's the tricky bit what value would that be we know the value of KW it's 1 time 10- 14 at 298 Kelvin remember stays the same any constant stays the same at a certain temperature but what about o minus well actually it's a little bit kind of I suppose easier yet more tricky in some ways because the dissociation is one: one um we can actually say that KW in this particular instance is equal to H+ squ because it's the same these two values would have the same value so H+ therefore in this instance at 298 Kelvin is equal to the square < TK of 1 * 10 -4 which means the H+ is equal to 1 * 10- 7 which means that pH is equal to seven which is what we' expect pure water kind of a room temperature we'd expect it to be neutral uh or should word that different our interpretation of neutral because things get a little bit weird because that for example 100° Celsius so what was that be uh 373 Kelvin uh we find that KW = 51.3 * 10-4 and when we work this out we do the exact same thing we find that the pH of water is 614 so 100° C the pH of water is 6.14 well that appears acidic and this is where the tricky thing is it's not acidic that is not an acid although it has a pH which to us normally associate we associate seven being neutral it's now apparently acidic it's not this is now the neutral point of water at 100° C 6.14 so if 100° cus water had a ph of 7 we would actually say that it's actually now alkaline if that kind of makes any sense a bit of a bizarre kind of concept it's not acidic because the H+ and the O minus cancel there is no overall swaying in either direction of H+ ions or H minus ions to therefore actually make it acidic this is the neutral Point that's the key thing the neutral point has changed and again to work this out I just did my KW = H+ s put 51.3 in mathed it up and I got 6.14 again two decimal places remember with all our calculations here so that's a bit about KW this this nice new sort of random thing uh that we've kind of I've gone off the page oh no that we've we've come across there well we can use that that concept quite nice and I'm going to change color for this I'm going to aim for salmon what color is salmon is it orange I really want salmony color color is a salmon oh yeah it's not really salmon at all but I'm going to go with it oh no no I'm not yeah what what color is a salmon is that maybe um what about if we were asked to find the pH that's a lovely color isn't it look at that like skin pH of 0.5 uh mole per decim cubed na oh well we know that pH is minus log of the hydrogen ion concentration but we're not dealing with an acid we're dealing with a with a with an Alkali remember that Alkali dissociate similarly to acids they associate into this boring ion in this case the sodium and they're now in exciting on hydroxide ion but that's not really helping because we can't put that into a into a pH calculation you can see things like Po but we're not using that you don't know worry about that certainly on on AQA anyway so pH is equal to minus log of h plus ions or the H+ ion concentration but look this guy we can use this we can use this KW idea here oh that's not really helped as I keeping these notes there we go we can use this to actually now incorporate or to form a relationship between hydrogen ions hydroxide ions and this KW CU this KW value at 298 kelv which we're assuming here sticks that has one time 10us 14 regardless so what we can say now is the I'm going to write out in full kw+ oh minus concentrations remember we know our value for this it's 1 * 10- 14 we don't know this one but we know our value for this is 0.5 because of our 1: one dissociation so I can work out therefore the H+ ions are going to be equal to or math C up KW ided the concentration of O minus ions if I do my maths there and I put my values in um I am going to find and I'm not going to bother working this out in long hand because there's sort of no real need to the pH is going to come out to be 13.7 note my two decimal places I am wonderful in that way look two decimal places got to keep that doing the question would probably say that if not you need to just do it anyway so 13.7 that's kind of what we' expect it's a high pH and this is a strong Alkali it's you know sodium hydroxide um so we'll do a couple of other examples now which if you want to pause these and then and then go through them you can uh and I'll run through them both as well so the first one is again looking for the pH of 6.14 in this case mole per decimeter cubed potassium hydroxide okay so again similar concept as before I'm going to skip straight to the rear Arrangement here um because this this guy is is dissociating just like the one before you can perap see where I'm going with this um or rather what the next one's going to be KW over oh minus same thing boom pH comes out this time look 6.15 is is a high concentration the pH comes at 14.79% it can go minus and it can go high if this was a concentration of 14 it would go even higher again it might even reach sort of yeah I think we perhaps even reach 15 odd there right last example is going to be and I'm not going to do any exam questions on this video either this is the last two I haven't done any the next one I'll probably do it tying in some of these some of these Concepts it's just this the exam questions are literally just what I'm writing here there's there's nothing else to do okay so final one is find the pH of 0.137 mole per decimeter cubed barium hydroxide solution barium hydroxide solution okay let's look first of all at dissociation because we've got this two we've got to be careful here and it's the same as the sulfuric hopefully you either I'm hoping you pause that if you want to way to work it out same isn't that weird the of time still going but me not the way time yeah at 2 oh minus we've got to therefore accommodate for the fact that this ratio of 1 to two means our concentration of this case our concentration of hydroxide ions would be oh 0 274 yeah .27 yeah 274 that's obviously we're going to change our value don't get tricked down to 0.137 same now same thing as before work it through find your H+ concentration eventually your pH is going to come out at 13.44% pH of of water then using that concept uh and the link between H+ and oh minus to actually allow us to find the H+ concentration in alkaline in basic basic Solutions uh and there you go so bit about uh KW bit about pH of bases uh hopefully that's been some help again if you any problems comment and all the rest of it uh and good luck with that