 So hopefully everybody's having an all right Monday morning, I guess. Remember exam three is on Friday, November 19th. So that's this Friday. So remember we pushed it back one week. Quiz four was returned today. So if you didn't get your quiz four back, it's probably because when I got to your name you weren't in the class yet. So if you could come by after class I'll give it to you. The average was about a 17 out of a 30. So it's lower than what I would have expected, but bodes well for everyone. So remember if you're online schedule your exam with the testing center between November 17th and well really 18th, the 19th you can come in and take it with us in class. If you want to do that make sure you email me and tell me. The exam will cover the end of chapter six to whatever we finished today. Remember there's the practice exam that's posted. There's the solutions to the practice exam that's posted. The quiz four is posted. The solutions are also posted to that. Remember that just because I post something like a quiz or something like that doesn't mean that those four problems or those five problems are the only problems that I could test you on. So it's clear to me that a lot of you guys decided that only six problems that were relevant were the ones that were on the practice quiz. Of course if you look at the two chapters seven and eight just look at the back of the book and there's like a hundred, a hundred and fifty problems. So if you're only studying five out of a hundred and fifty problems you're kind of what they say putting all your eggs in one basket and it's just not the best way to go about things. Of course those practice quizzes are quizzes that I gave last term. So just like you they had to study a lot of other stuff too. So don't just put all your trust in that practice quiz although it will give you an idea of what the quiz should look like. Remember we have a review session today after class 10 to 11. I emailed everybody about that this morning too but in that review session the first thing I'll be doing is going over this quiz four. So if you guys did as well as you'd like to on the quiz four then I would suggest you don't have to stay you know but if you did maybe a little less than what you would have liked to do maybe you should stay and figure out how to do those problems because of course those problems are going to be similar to problems to what you find on the exam. So that being said we can get started and get finished with chapter nine unless there's any questions. So I think it's on the syllabus. So that's for online people. Okay yeah so it's different. I think it's Wednesday right? Yeah I think it's Wednesday and I don't... Is there a time on there? Maybe it's just a schedule. December 15th. No so that's the week of the 13th. Okay so it's December 15th from 10.40 a.m. to 1.10 p.m. Okay so I'll give you... It'll be about an hour and a half long test but if there's nobody in here afterwards I'll give you more time if you guys need it. You know I won't make a really long test though or anything like that. Okay again if you guys didn't get your quiz four back it's because you came in after I found your name if you're one of those people come back come up and get it after class the average was about a 17. Okay so any more questions? Okay cool. Okay so recall the last thing we were doing last time was looking at this reaction of one water molecule with another water molecule. Let's write this mechanism up on the board. So we've got one water molecule. So remember how I was showing you about the arrows and the electrons and whatnot. So I'm just drawing the Lewis structures here. Okay so if we wanted to write the equilibrium constant for this reaction remember the equilibrium constant is going to be what the products over the reactants. Okay so in this case the products are H3O plus OH minus and we've got two H2O's. So because we've got an equilibrium arrow we can write an equilibrium constant for this reaction and when we do that the equilibrium constant is going to be remember it's the concentration in molarity okay so brackets around the hydronium ion times the hydroxide ion over water squared right but when we're writing our equilibrium constants we don't put for solids or liquids only for gas and aqueous. Of course H2O is a liquid right so this term will cancel out of the equilibrium constant so the new equilibrium constant for this reaction and we call it actually KW is going to be the concentration of hydronium ion times the concentration of hydroxide ion and remember if we switch the slide here that that combined thing KW equals 1.0 times 10 to the negative 14th okay and since we have half of it is this right if we divide this by 2 right we'll get 1.0 time I mean if we take the square root of that I should say sorry we will get 1.0 times 10 to the negative 7th molar for each of these concentrations here okay so why do we care right well it's because we can use that information to do pH calculations and of course if you've been looking at the practice exam you're going to have to be doing pH calculations for that so let's go over a pH calculation so it says calculate or in this case this is the concentration of hydroxide so it says calculate the concentration of hydroxide in a solution that has hydronium ion concentration of 1.0 times 10 to the negative 5th okay so what do we know we know that KW equals that and that equals 1.0 times 10 to the negative 14th what else does this give us well it says that we know the hydronium ion concentration so let's write that down 1.0 times 10 to the negative 5th molar right yeah so is this solution acidic or basic we'll talk about that in a little bit so we know KW we know the hydronium ion concentration and we're looking for this so that's cool we can just rearrange this equation around right so we get a hydroxide ion concentration so what do we got to do we got to divide both sides by H3O plus right when we do that that cancels there and divide by H3O plus here so that's going to be the new equation so the hydroxide concentration equals KW over H3O plus everybody follow me yeah and then 1.0 times 10 to the negative 14 divided by 1.0 times 10 to the negative 5th again I'll put this in like a parenthesis because these calculations won't give you the correct units because the term of equilibrium constant doesn't have any units okay so you can't just cancel out all your units or get your units out of this one okay so unfortunately I'll just erase that and we have to remember that because this says brackets around it that it's going to be a molar concentration okay so when we do that of course it's going to be 9 because 14 minus 6 equals 9 right so 1.0 times 10 to the negative 9th okay and then the question asks the OHR well it doesn't say oh yeah it says solution acidic or basic so if we look at we can compare the concentration of hydroxide to the concentration of hydronium right so if we look at that the concentration of hydroxide is smaller than the concentration of hydronium right because this is a smaller number 10 to the negative 9th is a smaller number than 10 to the negative 5th if this is smaller than this then we call it acidic if it's the other way around it's basic if they're equal it's neutral okay so this one is acidic the other thing you can think of is is this to the negative 7th or not right so if you remember these have to be equivalent to be pH 7 okay well we'll talk about pH a little bit more but water isn't acidic or basic that's one thing we do know right water isn't acidic or basic so if this is going to be smaller than to the negative 7th like it is here then it's going to be acidic if it's going to be bigger than negative 7th then it's going to be basic okay so you can look at it a couple different ways okay so what's all this acidic basic neutral stuff that we're talking about so all we're really talking about is the proton or hydronium ion concentration in these solutions okay so if you have a bunch of protons in these solutions or a bunch of hydronium ions protons and hydronium ions are the exact same thing essentially okay you can have a range of different acidities okay so depending on the amount of protons you've got so you can have a proton concentration reasonably to about 10 molar in solution and to about very small amount 10 to the negative 15 molar in fact you can have smaller amounts than this or bigger amounts than this they're just not commonly seen on the pH scale okay the pH scale goes from 0 to 14 so what you'll find is if you have a higher or lower than this number here you're going to get pH numbers of like 15 16 weird numbers okay or if you've got something higher in concentration than this then you're going to get negative pH numbers okay so that's strange too I don't want to worry about that so much because we're not going to be dealing with anything outside of this range of concentration in this class okay so how do I figure out what the pH of a solution is okay well this term pH of course a lot of quality control jobs deal with a lot of pH if you have ever been a lifeguard or anything like that or a pool guy or whatever I'm sure you've had to deal with pH or if you're a gardener or anything really you know you have to deal with pH so what does pH mean well pH okay so why is this a small pH why is that a big H I don't know okay so this P here this P that just stands for the negative log of something that's all that means this negative log of and H what do you guys think that means the proton concentration good job that's awesome yeah so that just means the proton concentration okay so if you look at what the equation for pH is negative log of the proton concentration right that's the exact same thing that just that little pH stands for okay so pH equals the negative log of the proton concentration so H sorry H equals which also equals okay so here's some like we said earlier if we've got pH that is greater than 7 then we call it basic okay if we've got a pH that's exactly 7 we call it neutral and if we've got a pH less than 7 we call it acidic we call it neutral at 7 because that's when the concentrations of the hydroxide ions and the hydronium ions are the same so they neutralize each other they cancel each other out okay so if we look here we can see some common pH's right so one molar HCl this is some stuff that you use in lab for a lot of you who don't like to wear goggles right you can get this stuff splashed in your eyes this is pH zero that would take out your eyesight pretty easily you know so you got to watch out about this stuff stomach acid is about pH 1 to 3 right if you've ever felt stomach acid right you can feel what a pH of 1 to 3 feels like pH 2.2 to the 2.4 vinegar 3.4 beer 4.2 about 4.2 unpolluted rain water 5.6 urine 4.8 to 7.5 milk 6.4 blood 7.4 seawater even more basic at about 8 10 milk of magnesium like peptobismol stuff right that's 10.5 why should that be basic peptobismol it's trying to soothe your stomach right so you got a bunch of acid in your stomach so you got to get something that's basic in there ammonia, lye and then one molar sodium hydroxide is 14 pH which would also burn your eyes out if you got it in there so you can see here this picture of the universal indicator so it shows the different pH's and you can have an indicator so some organic molecule that will bind to protons in this case the more and more protons you get the more and more red you get right so it doesn't look like the basic pH's it changes very much but once you start adding protons it goes from orange to red to pink or something this is a really good indicator to use I know we looked at that buffer lab a couple of weeks ago it might have been more ideal to look at it after this week but you are hopefully a little bit familiar with indicators by now because of that so let's try some of these problems so let's calculate the pH of a solution where the proton concentration is 1.0 times 10 to the negative ninth that's what that says right so what do we know? we know this equation and we know molar pH is another quantity that doesn't have units so all we got to do is to negative log times 1.0 times 10 to the negative ninth and I again don't put units there because it'll confuse you people will put units at the end even though it doesn't have units so the way I do this on my calculator is just type in well since this is 1.0 if it was something else I guess I'll just type in 1.0 just to show you guys I would normally just type 1 1.0 e negative 9 then push enter and then just take the negative log of that number and when you do that find that it's 9 there's a button called log so there's a button called log and if you just so most calculators have a positive slash negative sign that you can just push that button so you push what I always do is do the quantity first and then just push enter but if you like it you can just do it this way too you know so take this quantity like that and you can type it in your calculator that way too sorry this should be 9.00 the reason being is because when you take the log here it adds one more significant digit to your answer okay I'm not going to be too concerned about that addition of one more significant figure you should know that there should be at least two significant figures here though okay so if we were to ask ourselves is this basic or acidic hopefully everybody would say this is basic because the pH is above 7 so are there any questions about this problem so do you guys think you could do one on your own like this problem except with a couple different numbers so let's try this one okay 3.6 times 10 to the negative 4 so again there should be three significant figures on this calculation although if you don't put 3 you put 2 that would be fine with me so I got 3.44 for that answer so if you guys didn't get that answer I would check yourself yeah if you didn't get that answer it's probably because you're typing in something wrong you know like quantities wrong or something notice this notice this that the last one was the negative log of 1.0 times 10 to the negative 9th did you guys remember that and our pH was what 9 right okay so that's not a coincidence notice this one this one's about half way between negative 3 and negative 4 right so this should be about half way between 3 and 4 okay so this number should correlate something with this number okay so I want you guys to think about your answers don't be putting strange answers like I don't know I don't know some strange answer don't put like a pH of 26 or something like that okay do we need to do another one of these what's the pH of this solution so you should be able to look at that and do it real quick 4 right 4.00 actually but yeah it's 4 right yeah so HCl right is a strong acid so that's the thing I guess that's the other thing this thing this problem is asking us is to remember can I erase this stuff here yeah cool okay remember we have to know our strong acids so you guys remember what your strong acids were remember we talked about them on Friday all of the halogen acids right and then there's a couple other ones but HCl remember why do we call it a strong acid what does the strong acid do it completely dissociates right like that right so whatever concentration of this we have right what does it say one point that's the concentration of that if we wanted to know the concentration of this right we would have this and remember the chemical equation tells us the ratio of you know what these species are right so we got one to one to one ratio here so if we go one molar H plus one molar HCl right molar HCl cancels out giving us the concentration of H plus to be the exact same concentration as and then of course with that you can plug that into your negative log equation so everybody understand what I did then so remember this is stuff we learned in like chapter 3 taking the equation and doing ratios with it okay so here's another strong acid nitric acid HNO3 so if you guys didn't know nitric acid was a strong acid you might want to put that in your memory bank is strong acid so again the same thing here 2.5 times 10 to the negative fourth molar nitric acid so what's the concentration of hydronium ions we should have in this no the concentration of hydronium ions not the pH of this solution yeah you can tell me louder so everybody yeah 2.5 times 10 to the negative 4 right how do you figure that out point to where you could figure that out on the board everybody point to it yeah if you could just over there right over there okay cool so what should what two numbers should the pH be in between would you expect 3 and 4 right and hopefully you could do that without plugging that into your calculator okay and how would you be able to do that how would you be able to do that yeah how would anybody be able to do it how would you be able to do it not listening huh yeah who needs to listen right okay so since we've got this number here 4 right and since this is going to be between negative 3 and negative 4 we know that that number should be between 3 and 4 what is the number 3.60 is that between 3 and 4 yeah just like we would have expected okay so we can now hopefully calculate pH going forward okay so let's now calculate pH or calculate the concentration of hydronium ions give in the pH of a solution so can I erase this stuff here so just like the inverse of addition is subtraction and the inverse of multiplication is division the inverse of squaring something is the square root right the inverse of any or any mathematical function has an inverse of or also has its inverse okay the log system is no different okay so let's look at the equation that we had for pH okay so that's pH right what if I were to be given the pH well how do I get rid of this stuff negative log okay well what log really means is log base 10 okay so it's actually going to be the inverse of 10 to the whatever okay so let's try to do this right that's log base 10 right that's not negative log base 10 so in order to get the negative out of there we got to put it to the other side right so multiply both sides by negative 1 okay we get now negative pH right equals the log of the proton concentration like that and then what did we say again remember to get rid of log we have to raise everything to the 10th power right so 10 to the negative pH equals gets rid of log right so we could go 10 to that 10 to that that cancels that and that it's just the inverse okay sure you remember that from Algebra right well I mean what else what other math classes required to take before this class right just Algebra right college mathematics so maybe college mathematics is the one that you should have taken to remember this one okay either way who cares who cares if you've never seen it before this is the way that you do it okay this is chemistry chemistry class not really mathematics anyways okay so 10 to the negative pH equals the concentration of protons so the concentration of protons in this equation if the pH equals 5.92 okay is 10 to the negative 5.92 yeah not so bad why should it be the 10 to the negative 6 well the thing is the pH isn't 6 it's 5.92 but you're right you're trying to see somewhere in between negative 5 and negative 6 which is what is the answer something something something times 10 to the negative 6 right 1.2 and remember 3 goes back to 2 if you can remember that you know I don't really care about the significant figures on this okay I don't want you to just put one significant figure okay but be reasonable about things so that right but remember we also have to put units like that okay how do we do that in the calculator how did I do it I did like this in my calculator here 10 to the carrot yeah that carrot button it's the one that looks like a little hat and then I put quantity negative 5.92 so is everybody cool with that no you're still not getting it did you put it exactly like how I did it let me see I can't find it I lost it she said it's the that button yeah I'll look at it after class unless somebody else can show you during class so if you can't get this on your calculator you can come to me after class during the review session since you all will be here anyways and show me your calculators and we'll get it taken care of but the concept everybody understand the concept that's the important part okay what about this one what's the concentration of protons if you've got a pH of 6.00 what is it what should it be 1.0 times 10 to the negative 6 right yeah should be able to do that in your head after a couple of doing these a couple of times a couple of rounds you really should it really will click pretty quickly I think okay so that's how to get the hydrogen ion concentration that's how to get the pH from the hydrogen ion concentration what if I wanted to talk about the hydroxide ion concentration okay remember the pH doesn't tell me anything about that it just says the negative log of the hydrogen ion concentration right but we do know this other term KW equals the concentration of hydronium times the concentration of hydroxide alright and that equals 1.0 times 10 to the negative 14th KW so you can combine those two equations this one and this one and you should be able to get these answers okay so let's go through this calculate the OH concentration for a solution with pH 4.0 what's the hydrogen ion concentration of this solution yeah 1.0 times 10 to the negative 4 right okay you should you could already just be like well the hydroxide ion concentration must be 1.0 times 10 to the negative 10 right because negative 4 and negative 10 or 4 and 10 have to add up to 14 right but you can do it stepwise if you like to do a lot of work okay so all we got to do is use those two equations so it said that the pH 4.0 so well remember we can get the hydrogen ion concentration doing that so we got hydrogen ion concentration equals 10 to the negative pH which equals 10 to the negative 4.0 which is of course the same thing as 1.0 times 10 to the negative 4 but now we've got concentration unit molar okay and then we know this over here remember KW equals 1.0 times 10 to the negative 14th so if we're looking for the concentration of OH well that equals remember divide there so we got KW over the concentration of H plus concentration of H plus is that KW is that right I just I did it in my head but you can do it in your calculator so just think just think this well I mean I know that this is going to give you this number here because this remember we said is the equivalent to this this and this will correlate and this and this are going to equal should you know equal to something this plus this should equal that right that's how I did it in my head like really quickly way a long time ago and some of the other students did it in their head too but if you can't there's a stepwise process to do that so we don't all have to do it in our head that's the cool thing that's why we got calculators to do if you can't do this in your head right now I probably couldn't have done it in my head when I was sitting in your spot so if you can't don't feel like oh I'm missing something but it is possible I'm saying you know so don't let it be so daunting you know don't let it to be too daunting of a task to you now this one you can't really do in your head right because it's not pH equals integer okay so but this one's just 4.95 right so 4.95 there so now we've got to plug this into our calculator so 3 so this should be 2 like that you would have expected that to be in between 4 and 5 right and it is I mean you'd expect it to be closer to 5 okay and then here all we've got to do is take this and now let's plug this into our calculator the way I'll always do this I don't know if you guys do this already we've talked about this before I guess is to quantify the top number of 1.0e so quantity 1.0e negative 1.4 and then divide that by quantity 1.1e negative 5 where I plug this into plug this into my calculator and I got this number yeah so this is this number this going through this stepwise process okay and then I take this number and put it here because of this equation alright and then I get this number and then I box my answer and I go to the next problem to a lesser extent the OH concentration can be expressed similarly so the POH equals the negative log of the OH concentration so it's the exact same thing as the pH except opposite Pkw what do you think that means Pkw what does P mean what does P mean negative log of the negative log of the kw what's the negative log of the kw can anybody do that in their head what's the Pkw 14 yeah 14.0 or whatever that's the Pkw because that's the negative log of this number right what you'll find is that the pH plus the POH add up to equal 14 so since I see people are packing up and I'm not ready to go yet I'll just skip this problem because I need to get to something else but you can do this problem exactly the same way as you've done the pH problems and in fact we can go over this problem in the review session if you want there's some other P constant that you're going to want to know for weak acids they have a Pka and ka is just the acid constant so that's the equilibrium constant right so ka equals the keq of weak acids so it's not anything special the keq which is the concentration of products over the concentration of reactants okay and the P of the ka is just the negative log of that so here's a list of ka tables there's a list of pka so what you find is that if you've got this is a much more unwieldy number the ka just like the pH I could just say 1 2 3 4 5 6 7 whatever and somebody will know whatever pH it is right what if I say 3.0 times 10 to the negative 12 or something like that you've got to figure out now what the pH is right okay so if I just say pH 8 everybody knows oh that's basic okay that's what the pka does for you too okay so if I give you all of that the k the equilibrium constant of that acid is 3.2 times 10 to the negative 9th right that doesn't mean very much to us okay but what we find is that when we convert it to this pka unit that'll give us some like idea of how strong this particular weak acid is okay so what you find is that if you have a very low pka you've got a very strong acid you've got a very high pka you've got a very weak acid importance of pH and pH control agriculture crops grow best physiology blood pH shift of 1 pH unit is going to be fatal so up or down acid rain of course sewage treatment industry and I don't think we'll get to buffers so you can cancel the buffer questions off of your practice exam okay if you didn't get your quiz Stephanie Garza come down and yeah the review session in like 2 minutes or a few minutes