okay uh welcome back uh what we're going to be doing is the laboratory number eight which is the the determination of the ka of an unknown acid now to determine the k of an unknown acid the what we're going to be working with is a concentration of a known strong base sodium hydroxide and this is going to be our unknown acid we're going to be using in this case the ph meter and the ph meter is going to be telling us what we know as the ph of the solution now we're not if we're accustomed to saying that the ph when it's equal to 7 that it's neutral well in this case since we're working with a weak acid weak unknown acid and a strong base the ph equally into seven doesn't mean it's reached its initial point the equivalence point is actually in this case going to be a lot higher because we have a strong base and a weak acid so the interaction of the strong base is overpowering the acid so therefore the ph the equivalence point which is what we're looking for is a lot higher than a ph of 7. so in this lab we're going to be recording the ph as i'm adding the we're going to be putting the ph as i'm adding the base to the unknown acid and so this portion here in the p of the ph meter is actually very important so we'll be you'll be replotting each of these uh the ph as it's increasing every time i'm adding a about half a milliliter of the base no of the concentration known base to my beaker every time i tell you to record the ph that's the part that you have to record it on your notebook and then translate that into a graph so you can observe where that equivalence point is uh where that equivalence point is at so first of all what i want to do is get my concentrated sodium hydro my known concentration starting hydroxide okay this is my stock solution and i'm going to go ahead and put it into this buret now to make sure that the buret that i keep the solution here i make sure that my stopcock is perpendicular to the burette so this makes sure that it's pouring in the liquid it stays in here and nothing runs out also when we're pouring uh basis sodium hydroxide we don't do it above our head we make sure to remove the buret from the buret plant and bring it down lower eye level and go ahead fill it up using a funnel ensure that it doesn't spill all over the place then as i'm getting closer to the zero mark i go ahead and remove the funnel and i'll start to use plastic pipettes so this instrument is actually very precise so i shake it off a little bit i'm going to add a little bit more the reason why i'm adding a little bit more of my sodium hydroxide is because what i want to do is i want to go ahead and kind of fill this tip with the sodium hydroxide solution because right now i have uh well it's air in there perfect i have no air bubble in my tip and i'm at the zero mark so burets are interesting in that we're so accustomed to reading the volume starting from zero closer to the baseline and moving and as you're going up the glass where the volume numbers are increasing well burettes are different in that you start from zero on the top and the volume is increasing as you're going down all right so all right okay so what i'm going to do now is i'm going to take 10 milliliters of my unknown weak acid put it in that graduated cylinder just to be as precise as we can and i'm going to put it into this beaker [Music] so the speaker has my unknown acid and here i have 0.1 molar sodium hydroxide what is shown here so that's already been standardized what i'm going to do now is take a stir bar this little white pill looking thing and going to go ahead and put it into my acid solution acidic solution go ahead and remove this water it's distilled water which is good then i'm going to go ahead and put it into the the acidic solution now we know that this is acidic solution because the ph meter is reading the solution to be at 2.8 we're going to go ahead and let it stabilize for a little bit it's going to be at 2.8 no the ph is still dropping but when the ph is less than 7 we know that is an acidic solution so yes this is acidic but i don't know the concentration of it i can figure out the concentration by using the known concentration of this whatever's inside the this buret all right so now this we see that the ph is holding at 2.69 okay i'm going to go ahead and start stirring it all right okay the reason i put the stir bar in there is because i want to i want to make sure that as i'm adding the base into the solution it becomes homogeneous in the the solution is homogeneous it doesn't just stay concentrated localized in one area so we're seeing that the ph is around 2.6 we're going to call this my estimated digit the last one 2.66 is my estimated digits now let's see what happens when i add a little bit of the base so start it off at zero i'm going to add a couple of drops the ph is increasing so i'm going to go ahead and let it uh equilib let this ph meter equilibrate so i went down by 0.5 mls now what we're looking here is we want to reach the endpoint that we're looking for is above a ph of 9. so initially if when we look at the when we plot this out we have our volume when we plot this out we're going to have the volume of our known sodium hydroxide on the x-axis and the y-axis that's where you're going to be plotting out these values the ph values so what we're trying to do is replicate this graph that we are showing here on your example in your lab manual okay so we stopped at 2.9 have we reached the end point no so we're going to go ahead and add one more or 0.5 ml more of sodium hydroxide okay we roughly have 1 ml so at the 1ml i have a point ph of 3.3 so now the volume of the sodium hydroxide in the buret is at 1.5 i'm letting the ph meter equilibrate or stabilize okay so the ph at 1.5 is going to be 3.6 3.60 let's go ahead and add 0.5 mls more so now the volume of the sodium hydroxide is at 2.0 milliliters we wait for the ph meter to stabilize we're seeing that the ph meter the the values are increasing indicating that the sodium we have sodium hydroxide in this or we have hydroxide ions present because as the ph is increasing that means the acidic concentration is dropping but we haven't reached our endpoint remember we're looking for this ph to be at above 9. okay so uh at the volume of 2.0 the ph of our solution is 7.78 i'm sorry 3.78 the volume of the ph is 3.78 and now we're going to go to 2.5 okay waiting for this to equilibrium [Music] okay so a volume of 2.5 the ph is at 3.89 increasing the volume of sodium hydroxide adding more sodium hydroxide so at a volume of three milliliters 3.0 milliliters of the 0.1 molar sodium hydroxide solution the ph is now going to be at 3.97 so a volume of 3.0 sodium hydroxide i have a ph of 3.9 3.98 is going to be the ph okay so now i'm at a volume of 3.5 the ph is still increasing okay so a volume of 3.5 the ph is 4.10. okay now i'm at a volume of four milliliters so okay waiting for this to stabilize perfect all right so at a volume of four milliliters the ph is 4.17 okay still adding some more there we go 4.5 milliliters ready for this so at a volume of 4.5 milliliters we see that the ph is still increasing very slightly we don't see that huge spike that we're looking for to tell us that we've reached the end point okay waiting for this the ph meter to read well to stabilize and once it's stabilized we usually see here on the bottom right corner of the ph number is the word said ready okay still dropping still not stabilized so when you're doing this experiment what you want to do is you want to make sure that you're as precise as you can so precision is a or yeah precision and accuracy plays a huge role especially when doing these uh analytic analytical techniques all right it seems like we're getting closer to stabilizing we're going to go ahead and say that that volume of 4.5 the ph is 4.30 i'm going to add an another half a ml so excellent all right so the volume of five milliliters i see that the ph is still at four point three 4.30 actually 4.31 we're gonna go ahead and increase add some more sodium hydroxide we now have a volume of 4 5.5 milliliters of sodium hydroxide [Music] at a volume of 5.5 milliliters sodium hydroxide the ph is 4.33 [Music] okay okay another 0.5 so what we're seeing here is kind of that plateau that way from the in that graph that you have in your lab manual we see that the ph starts to increase as the volume of sodium hydroxide is increasing we've hit that plateau and that's kind of the area that we're seeing that the numbers are being that we're repeating the ph even though the volume of sodium hydroxide is increasing so we might be getting close to the point where we're going to see a huge spike leading to a leading to our end point all right so at a volume of 6.0 milliliters we have a ph of 4.51 4.51 okay okay at a volume of 6.5 the ph is 4.53 nope never mind okay so the ph is increasing so that 4.53 isn't the accurate ph at a volume of 6.5 okay so at a volume of 6.5 the ph is 4.60 4.60 so now we're at value we're at a volume of seven milliliters 7.0 milliliters [Music] okay so at a volume of seven milliliters the ph is at [Music] 4.68 okay we're now at a volume of 7.5 milliliters [Music] at a volume of 7.5 milliliters the ph is at 4.75 [Music] volume of eight milliliters so okay so the volume of 8 milliliters we have a ph of 4.84 we're now at a volume of 8.5 milliliters so at a volume of 8.5 milliliters the ph is at 5.04 so now we're at a volume of nine milliliters okay okay so at a volume of nine milliliters the ph is at five point eighteen okay we wait for a while so now the volume is at 9.5 milliliters [Music] yes okay so at 9.5 milliliters the ph is at 5.50 [Music] now we're at a volume of 10 milliliters of the 0.1 molar sodium hydroxide [Music] [Music] excellent okay so at 10 milliliters the ph is equal to 6.35 excellent perfect so at a volume of 10.5 milliliters we see the ph has surpassed the ph of 9. so this is our endpoint this is the first trial we're going to do two trials however this first trial we were being very uh we were decreasing well we're introducing the volume of the sodium hydroxide in small increments the reason why is because i'm trying to find what i call the uh the sweet spot where i can just drain the sodium hydroxide until i get close to the equivalence point to kind of narrow down the range because right now i'm going by 0.5 increments so in the second trial i'm going to i'm going to go ahead and just drain the sodium hydroxide until 9 milliliters and then slowly uh introduce small increments of sodium hydroxide no longer of 0.5 milliliters i'm going to be introducing maybe drops of sodium hydroxide until i see that huge uh that huge jump so this is really good so i'm just going to go ahead and make a note of this oops so volume of [Music] 10.5 millimeters so that's where i want to i definitely want to repeat this experiment so let's go ahead and do it [Music] to do that i'm going to stop the stir bar just rotate the ph meter remove the stir bar rinse it off [Music] rinse it off and go ahead and dry it off so my first experiment and i want to make sure to rinse off the ph meter [Music] okay once it's rinsed off just gonna go ahead and dry it off dab it dry and get ready for my second trial where i'm going to take again 10 milliliters of sodium hydroxide solution i mean 10 milliliters of my unknown weak acid [Music] fold in the beaker go ahead and try this area okay what i want to do also is refill my buret the reason why i want to film refill my burettes because i want to make sure it starts off again at the zero mark okay there we go i'm going to take my stir bar okay i'll make sure i can get the drops in there all right so this is trial two we're going to have 10 milliliters of my unknown weak acid in here and i have refilled my buret to 50 milliliters of 0.1 molar sodium hydroxide so now instead of going 0.5 milliliters at a time i'm going to go ahead and do 2 milliliters at a time i see that the ph of the solution with is initially at 2.64 i'm gonna go ahead and add two milliliters okay i'm going to allow the ph to stabilize the ph mirrors okay yeah so right now in trial two at two milliliters the ph is at three point five nine okay so it was holding at 3.59 for a while but it jumped back up so 3.59 hasn't isn't the sta the it's not stabilized at 3.59 you see that it went up to 3.67 let's go ahead and just cause this to agitate a little bit faster awesome okay seems like it's holding that 3.76 yes so at the two milliliters of sodium hydroxide i'm the ph is equal to 3.75 i'm gonna add another two milliliters okay now i have a volume of four milliliters so even though i'm taking these uh larger quantities i'm introducing larger quantities into the beaker the data is very similar to what i did in my first trial where i saw even that two milliliters of sodium hydroxide in this unknown acid the ph was around three it's at a volume of four milliliters i saw that the ph was approximately around uh four so great so at a volume of four milliliters uh sodium hydroxide in the buret or force four milliliters used in here the ph is equal to uh six point i'm sorry four point one six so four milliliters sodium hydroxide ph is equal to 4.16 okay so i'm gonna do again another two milliliters like one two four six milliliters [Applause] so okay waiting for it to stabilize six six milliliters sodium hydroxide introduced into the uh unknown acid weak acid it's all steady nope okay all right we you can see that at the volume of six milliliters the ph is 4.61 we're gonna go ahead and add two more milliliters now i'm at a volume of eight milliliters [Music] so so okay so uh eight milliliters we have of eight milliliters the ph is four point nine seven so instead of adding two milliliters since i'm getting closer to where i saw the ph from my first trial uh jump up as i'm getting closer to point uh 10.5 milliliters instead of doing these large jumps i'm just going to go ahead and take it to one ml at a time so i'm going to go ahead and introduce 1ml total total volume of 9 milliliters so even at nine milliliters we see that the ph is around the five area still hasn't stabilized so i'm just waiting waiting for the ph meter to stabilize okay so now i'm at the nine meter i'm really close to nine milliliters of sodium hydroxide is in the uh beaker so at nine millimeters nine millimeters milliliters i'm sorry of sodium hydroxide the ph is equal to 5.27 now what i'm going to do is i'm going to take it slowly instead of doing large gaps of 0.5 i'm going to maybe do quick turns just to make sure that i can see exactly where the point where i see that huge spike in ph two three okay so this is where i'm going to be having to write great so now at nine point one so at 9.1 ml the ph is reading at 5.36 gonna make a little line there sorry about that i think i better use a whiteboard okay so now i'm going to go ahead and do a couple more turns one two three so now i'm at a volume of 9.3 they have a ph of 5.43 so oops great now i'm at a volume of [Music] 9.5 the ph is reading at 5.62 5.61 okay so this is the ph so now the volume of 9.7 milliliters the ph is five point seven five cool because i'm really close 9.9 milliliters oh the ph is at 6.1 6.18 so now i'm excellent i just introduced one one drop and that's going to give me a volume of 10 milliliters so in the first trial we saw that at 10 milliliters it was already starting to we're starting to see that sudden increase in the ph so by doing small drops at a time we're kind of really honing into where we can see that the sudden spikes instead of having large so instead of having a large peak we're focusing on a very certain very specific specific area so at 10 millimeters we have 6.60 so at 10.5 that's where i saw the sun uh that's where i saw the end point now i'm at ten point one milliliter at 10.1 [Music] i'm already reaching i've surpassed the ph of 9. i'm at 9.27 point three zero so between nine point five and uh ten milliliters that's where we see that that sudden point of inflection the large point of inflection change inflection and this is going to be nine point four three so again this is ph and we can do one more drop just to really solidify that i've reached the end point 9.48 let's erase it 10.2 at 10.2 i have a ph of 10. well we've definitely reached our end point because it's above a ph of 9. ph of 10. 16. 10.16 so at the 10 milliliters or 10.2 milliliters we have a ph of 10.16 so to prove that it's actually that we reach the endpoint we can use our color indicators our color indicator is going to be penetrating if i reach that end point it should turn the solution so the color indicator is telling me i'm above a ph of eight and the ph meter is definitely showing that i'm definitely above it exactly uh what ph number i'm at which is at ten point ten point zero excellent so this concludes the experiment thank you