5/10/2021 pH calculations

Added: 2026-05-12

[00:00:08]all right hey guys good morning um so kind of picking up where we left off thursday um so looking ahead this week remember we are finishing up our last unit this week um so today we are looking at the ph calculations and stuff so i would make sure to have a calculator handy while we're doing notes all of the formulas for acids and bases and all the calculations are are on your reference sheet so it's going to be kind of figuring out which equation to use plug in solve for the missing variable solve for x tomorrow we are doing a lab looking at different indicators and the colors that they change we are going to finish up notes on the different types of acids and bases on wednesday we'll do a quick review thursday and then your last test for me before your final is on friday i'm also working on um [Music] i guess kind of a replacement test for my guys that bombed the stoichiometry test to kind of get up to a 60 so at least to possibly get up to passing what i previously had used flash so i'm having to rework it since slash went away so i'll be coming up with something that can be used to kind of replace that test grade if you made below a 60.

[00:01:50]so i know there are some people that didn't turn in the short answer they left the short answer blank things like that so i'll kind of give you an option to get that grade up a little bit um and remember so today is the 10th first period exam is on the 20th so that's next thursday so your last test for me is friday we'll kind of have some review makeup time next week and then your test is next thursday for first period uh miss kincaid's fourth period that is watching y'all's exam is on the 26th um so just kind of know that that's coming up there will be some review stuff posted um that you can go through individually or do some individual practice um but we'll kind of do a quick group review each day we'll look at some of the certain topics we went over um just to kind of refresh but a lot of that is going to be kind of makeup work time or that kind of independent review time we're still missing people but we'll go ahead and get started all right so where we left off thursday is we talked about i'll go back one slide so we talked about how water can self-ionize and split into hydrogen oxygen or hydrogen and hydroxide um and the aqueous solutions where hydrogen hydroxide are equal are neutral solutions like water so that's kind of where we stopped on thursday um well sorry i let somebody in there we go so one thing that we deal with with the hydrogen hydroxide in particular using that self ionization of water is we can do calculations using the ion product constant of water um so the ion product constant is 1.0 times 10 to the negative 14.

[00:04:06]so that's the product of the concentration of the hydrogen ions times the concentration of the hydroxide ions are always going to equal 1.0 times 10 negative 14.

[00:04:19]so this is a formula that is given on your reference sheet the constant for water is this kw kw is the 1.0 times 10 to the negative 14th how we find that kw value is by multiplying the concentration of hydrogen so anytime you see those kind of square brackets around something that's the concentration or molarity that goes in there so we have that concentration of hydrogen times the concentration of hydroxide equals the 1.0 times 10 negative 14.

[00:04:59]this equation could be used if you know the hydrogen ion concentration you can use this equation to solve for hydroxide ion concentration or vice versa bless you so this will be used to go from one ion to the other shhh um hmm i'm trying to pay attention by writing but if i'm going too fast for anybody let me know okay here's what one of the problems might look like that we can use that ion product constant from water and here it says colas are slightly acidic if the hydrogen ion concentration solution is 1.0 times 10 a fifth molar what's the hydroxide ion concentration of this solution now i'm just going to write our formula up here so looking at what it gives us it tells us that the hydrogen ion concentration is the 1.0 times 10 a fifth so we can plug that in and we're looking for the hydroxide ion concentration or that oh negative is 1.0 times 10 negative 14.

[00:08:30]so like i said all of these are pretty much plug in solve for x so whenever we're trying to get that hydroxide the o h negative by itself we need to divide by the 1.0 times 10 negative 5th on both sides and that will give us the concentration of o h negative now this is not particularly complicated math but remember the numbers in scientific notation if you don't type them into the calculator a certain way it tends to give you the wrong answer so remember you can use the ee button or put these in parentheses whenever you type them in that way the calculator understands that those are two numbers and not a 1 and a 10 to the 14. so it understands that those are numbers together so if we do 1.0 times 10 negative 14 divided by 1.0 times 10 negative 5th that gives us an answer of 1.0 times 10 to the negative ninth and concentrations are molarity so anytime you have those brackets remember it's a concentration so the unit is molar that capital m take apart the tissues out are the tissues out there's another box in that drawer right to the left of it [Music] good morning all right um so in terms of looking at what those numbers for the hydrogen concentration and hydroxide concentration mean so looking at those numbers we can determine if something is acidic or basic or neutral so we already said neutral is whenever hydrogen hydroxide are equal to one another um an acidic solution so acids are going to be where the hydrogen ion concentration is greater than the hydroxide ion concentration so that means the hydro9 concentration is greater than 1 times 10 to the negative 7.

[00:12:28]now a lot of people get messed up with these numbers in scientific notation so if the number is greater that means it's going to have a smaller exponent so anything to like the negative third negative fifth is gonna be acidic whereas a basic solution is going to have more hydroxide than hydrogen so the hydrogen ion concentration is going to be less than 1.0 times 10 negative 7. so that means that we're going to have a larger exponent so anything to like the negative ninth or negative 12 is going to be more on the basic side in terms of the hydrogen ion concentration basic solutions might also be referred to as alkaline solutions so if you've ever drank in or seen someone drink like alkaline water it's water that's slightly basic um that's for people that have like acid reflux and stuff like that to help neutralize the acidity in their stomach you and whenever we're looking at ph so acidic acidic or basic we can determine by looking at the h plus or oh negative but a lot of times we will actually change it into ph that way we have a little bit of an easier number to deal with um so a solution with a ph above seven is going to be basic so this can be anywhere from like 7.1 all the way up to 14.

[00:16:11]so 14 is kind of the top range of our ph scale a solution with a ph below 7 is going to be an acid so that can be anywhere from it can go all the way down to zero up to 6.9 and then neutral solutions truly have a ph of seven so that would need to be like 7.0 to be deemed neutral so the easiest thing is anytime you're looking to see if something is acidic basic or neutral find the ph that way you can kind of compare those numbers all right so how we find that ph so we can actually compare that number the formula for ph and this is on your reference sheet as well is that ph is equal to the negative log of h plus so the one thing is do not forget this negative sign or else it gives you a negative ph and we can't have negative phs and so you're using the log button on your calculator so you literally just hit the negative sign log and it opens parentheses for you to type in your hydrogen ion concentration so on ti calculators the log should be in that like left side column if you have a different type of calculator other than the ti or texas instruments ones we may have to figure out where your log button is if you're using the desmos calculator and stuff it also has the log button now there is kind of a shortcut whenever you're finding ph because this is pretty much just plug in solve for x if the hydrogen ion concentration is written in scientific notation and the coefficient is one so if it's 1.0 times 10 to some exponent then the ph of the solution is going to equal the exponent with the positive sign so just change the negative to a positive so for example if we have a solution with a hydrogen ion concentration of 1.0 times 10 to the negative second it's going to have a ph of 2.

[00:19:11]you can only do that when that coefficient is 1. if it's anything other than 1 you can't do the little cheat shortcut so if that coefficient's 1 the ph is equal to whatever the exponent is as a positive number shhh it's all right um whereas we can calculate ph we can also calculate poh so ph looked at the negative log of the hydrogen ion concentration so it measured how much of the hydrogen ion concentration we have poh is the same formula but it's dealing with that hydroxide ion concentration so this is also on your reference sheet is that poh formula and the poh scale is the reverse of the ph scale so we said in terms of ph we had neutral was seven anything below seven was an acid anything above 7 is a base whereas for the poh scale 7 is still neutral so that didn't change but for poh those smaller numbers are going to be basic and those higher numbers are going to be acidic so the ph and poh scales are reversed um there's also a formula on our reference sheet that will let us change so kind of like we were able to change with the ion product cost of water we were able to change from hydrogen to hydroxide there's also formula on our reference sheet to let us go between ph and poh that's this third bullet here this is what's on your reference sheet the ph plus the poh are always going to add up to equal 14.

[00:22:49]so the next two bullets are just that same formula kind of rearranged to solve for ph or poh but it's still one of those things that we're plugging in whatever value we knowing or whatever value we know and solving for x okay so here's what a problem might look like so here it wants to know what's the ph of a solution with a hydrogen ion concentration of 4.2 times 10 near 10th molar so part of the acid-base calculations is going to be figuring out which formulas to use because their total of seven formulas deal with acids and bases so here it's looking for ph and the ph formula is the ph is equal to the negative log of h plus it did give us h plus so we can plug straight into this formula since it gave us h plus so we are going to be looking for the ph is equal to the negative log of 4.2 times ten to the negative tenth so i said just don't forget to hit that negative button first or else you'll end up with a negative ph so here we should get a ph of 9.38 so we say concentrations have the unit of molarity ph and poh do not have units you're going to put units for the concentrations but not units for ph or poh so okay all right but let's look at this one this one wants to know what's the poh of a solution with a hydrogen ion concentration of 1.0 times 10 to the negative eight so sometimes you're going to come across problems where you may not have all the information you think you need so looking at poh our formula for p o h is p o h is equal to the negative log of o h negative did it give us oh negative no so we can't plug this straight in to get poh so we're gonna have to do a little bit extra work now what it did give us is it gave us that h plus h plus is in two different formulas we can either plug h plus into the ion product constant formula or we can plug h plus into the ph formula so there is more than one correct way that you could get the answer here [Music] yeah so based off of h plus we can plug it into either of these formulas so we can plug it into the ion product constant to plug or to solve for o and then plug that into poh or we can find ph and then remember there's another formula that can go from ph to poh either way you do it you should get the same answer it's just kind of picking a path and going with that path so do you guys want to use the ph formula or the ion product constant formula or does it not matter i personally would probably go for ph that's just kind of my preference i tend to avoid the ion product constant unless i have to use it simply because remember if you don't type those numbers in scientific notation a certain way it tends to give you the wrong answer so if we solve for ph if we go down this path so the ph we said was equal to the negative log of the 1.0 times 10 to the negative eighth and here's where you could use that shortcut so remember we said when that concentration is 1.0 times 10 to some exponent the ph is equal to that positive exponent so this is going to be a p ph of 8.

[00:30:40]and then we remember also have that formula where ph plus poh equals 14.

[00:30:55]so if we plug 8 in for ph so 8 plus poh equals fourteen whenever we solve for poh it should be equal to six so some equations you're going to have everything to plug straight in other equations are going to be two-step problems where you first have to solve for one thing to plug in to solve what you're looking for like i said for these ones that are two-step there's more than one correct way to get the right answer [Music] all right there's one other kind of conversion we haven't talked about yet so we found ph and poh whenever it gives us h plus or oh negative but what about going the other direction where we're looking for h plus or oh negative from ph or poh so we can use the antilog to solve for the concentrations instead of ph and poh so this is just the rearranged formula for ph or poh which is going to it's rearranged to solve for that concentration so the anti-log is on your ti calculators it's the second log button which right above log you have this 10 x as the second function that's the anti-log both of these formulas are also on your reference sheet so they're there for you you just need to be able to use them so if we're solving for the hydrogen concentration we can do the antilog we can do 10 to the negative ph and yet again don't forget that negative sign that's one of the most common mistakes and people get wrong answers because they punch in the calculator wrong if we're solving for the hydroxide ion concentration we'll do 10 to the negative poh so i do have another example but i'm gonna add a slide real quick um just so i can kind of show you all of our conversions so the things that we solve for we have ph poh we can also solve for h plus and oh negative so remember we can go back and forth between all of these and sorry i can't draw straight lines so remember to go between ph and poh we're using that ph plus poh equals 14.

[00:35:39]so that's the equation we used to go between those two to go from hydrogen to hydroxide we use that ion product constant for water the h plus times oh negative equals the 1.0 times 10 negative 14.

[00:36:09]remember going from h plus to ph is the negative log of h plus the opposite direction going ph to h plus is that 10 to the negative ph going from hydroxide to poh is the negative log of oh and going from poh to oh is that 10 to the negative poh so that's kind of all our formulas and when to use them so this is a diagram that you can kind of sketch for yourself and it kind of helps you go through it especially helps you figure out what steps you need to take if you're doing those multi-step problems so if it gives you hydrogen and it wants poh you can't go straight there so you either have to change into ph and then poh or oh and then poh same thing happens if you have hydroxide the oh negative and you want ph you either have to change into h plus to solve for ph or you have to find poh to solve for ph so this can kind of help you visualize what you need to do on those multi-step problems as well like i said you don't have to memorize any of these formulas all of them are on your reference sheet on your reference sheet under the formula page they're the last seven equations at the bottom of that formula list so it's just figuring out which equation you use to solve for what you want but you have all the formulas in front of you so here's an example um the ph of an unknown solution is 6.35 what's its hydrogen ion concentration so we said to solve for hydrogen ion concentration that was the h plus was equal to 10 to the negative ph and it gave me ph so i can plug that straight in i don't have to do anything extra so if we do 10 raised to the negative 6.35 so remember on your ti calculators that's the second log button is the 10 to the button so make sure to do the negative 6.35 so here we get a hygienic concentration that is four point let's do four point five times ten to the negative seventh now if you had done the like 4.47 that's okay and remember anytime there's the brackets anytime it's a concentration our unit is molarity so hopefully that wasn't too bad it's just really figuring out which equation you need to use and plugging in and solving for x sorry my voice is going out so let's look at this one this one's going to be a little bit harder um it says the poh of an unknown solution is 2.31 it we want to know what's the hydroxide ion concentration of the solution and what's the ph of the solution so it's really just taking it one question at a time so the first one wants to know what's the hydroxide ion concentration so remember hydroxide the oh negative is equal to 10 to the negative poh which it did give me poh so that one i can plug straight in so if we do 10 to the negative 2.31 that gives us a hydroxide ion concentration and that's going to be equal to now the calculator gives you point zero zero four nine now you can leave it like that and that is the molarity but remember especially on multiple choice and stuff a lot of our stuff will be written in scientific notation so this goes all the way back to unit one where we had to write numbers at scientific notation so remember we can only have one digit in front of the decimal so that would be 4.9 times 10 in order to get the decimal after the 4 we have to move the decimal three spaces that's the negative third molar either way that you have it written will be correct but understand if this was a multiple choice you might need to look for one or the other so don't let that stump you so we have our hydrogen ion concentration and now it wants to know poh or excuse me ph so we have options and i'm going to flip back to our little diagram for a second so we're looking for ph and we have oh negative or poh so we could change oh into h plus and ph but that would be a two step where we have poh so we can do a one step problem using that ph plus poh equals 14 in order to solve for ph if we're using that formula since that's going to be a one step so ph plus poh equals 14.

[00:43:37]plug in we know our poh is 2.31 so if we subtract to solve for ph that's going to be 11 and remember ph and poh do not have units so hopefully still not terrible like i said the math is not bad it's just figuring out which equation you need to use and life said there may be more than one correct way to solve for it but there may be a short way and a long way so either way that you solve for it though you should get the same answer all right last thing i want to introduce to you and don't don't draw this chart um this is just to give you kind of an example and this is something that we're going to deal with with lab tomorrow but we do deal with acid-base indicators so we already talked about when we were looking at properties of acids and bases that they changed color with different indicators and we talked about the lithmus paper and the phenolphthalein those are not the only indicators we come across indicators help us measure ph by changing different colors in solution so each different indicator has a ph that it will switch from one color to another so whenever we see that color change we know we've kind of hit that target ph um there are some that only have one color change there are others that have multiple color changes we're going to deal with one of those tomorrow that's referred to as a universal indicator and you'll see it can be anywhere from yellow to orange to green to blue to purple depending on what it's mixed in so there's some that help us gauge ph a little bit more than others but these are going to help us know kind of where that switch is from one color to another based off of that ph um so one that we'll use tomorrow is we'll use this bromine crystal green and you should notice it change from a yellow color into a blue color whenever we mix it with different solutions we're also going to use we're going to use phenol red which is close to the methyl red so we'll see it go from like a red to a yellow color um the phenyl phalenes also on here that's one of those that we talked about last week with properties where with acidic things it's that clear colorless and then it turns a bright pink color so this is something that's going to be similar to your analysis for your lab tomorrow so for our lab tomorrow we're going to have different solutions and these are going to be household things like lemon juice soda ammonia drain cleaner things like that and we're going to test the ph with the lithium paper strips and then we're going to mix them with different indicators and note the color change based off of the data you collect you're going to draw a chart similar to this that shows the ph and the indicator and you're actually going to draw a bar and color it or shade it in a certain way depending on if you have colored pencils or not to show what it changes or how it changes from one ph to another so i can't help you walk through that tomorrow but this is a chart that you can kind of refer back to to help yourself out um lab tomorrow we're going to be doing the same kind of thing we're going to have one group stream to everybody at home while everybody else kind of does their own and this one depending on how efficient and how quick you are it may take the whole lifetime to get all the data it might not it depends on how well your group's working together for your homework so what you can do on your part two homework that we started last week so last week let me zoom in so you can see a little bit so last week you could only do numbers like one through four and there's no five for some reason i just realized that so you can do numbers one through four last week if you scroll down so skips six through eleven go down to those acid-base calculations so the last part of this page and then the second page is all the stuff dealing with the acid-base calculations now make sure to show your work you don't have to show me that you divided multiplied or anything like that but i need to see your formula and your setup if there is no work and you just put an answer down you don't get credit some of you guys have realized that you just put answers and you don't get credit for it so you need to show your setup so what formula you use what you plug in do it on a separate sheet paper that way you can show it there is this chart on the next page just draw this chart and you can even leave a little bit of space to show how you're solving for h plus how you're solving for ph how you're solving for poh and depend determine if this solution is acidic basic or neutral so remember neutral is only 7 on the dot anything other than 7 is either acidic or basic so this will take a little bit of time to do but it's not terribly hard but these are calculations that you have to be able to do guaranteed you are going to have calculations like this on your test and on your final so make sure that you're comfortable with it so and remember the homework this week this homework is not due until thursday so we will finish up the notes on wednesday that's that numbers like 7 through 11 that we kind of skipped over so then there won't be much left for you to do after wednesday so in theory you could probably turn this in wednesday but we gave you an extra day um so you have the rest of lifetime we only about 10 minutes left in lifetime but then we have 25 minutes left in class utilize your time to go ahead and start working through some of these remember we'll get started with lab tomorrow my guys at home make sure that you log in on time tomorrow um because as soon as it hits the 7 30 in the morning i'm gonna let them start working um miss kincaid's fourth period guys that are listening make sure you log in on time because as soon as live time starts they're gonna start doing data so make sure you're in on time um and i will see you guys tomorrow bye guys