pH of a Weak Acid

[skateandtennis]

Could of used an ICE table... Simplifies EVERYTHING..

[jenbuxton]

Wooow, I can't believe I actually understand this -- finally!! I don't think my chem lecturer could have made it any more complicated... Khan Academy makes life so much easier.

[indian310]

How do you exactly find the KaHF that he did at 6:30? Does he explain this in any of the videos?

[shahruk10]

how is a pH of 1.4 a weak acid ?? :S

[imalazykid]

@shahruk10 That's because Sal has his pKa value wrong. pKa of HF is 3.15.

[shahruk10]

@imalazykid thanks i realized later on :D

[ihofaerefa]

you have a strange accent...lol completellyyyy

[asmcriminaL]

I got confused as hell, when you had the scientific notation, and had the x^2 stuff. You used an "x" in the scientific notation, I thought it was a variable, but it just meant multiply.

[DENlERU]

Can't you just say that [H⁺]=[F⁻], and [H⁺][F⁻]=7*10⁻⁴*1.5 and thus [H⁺]=sqrt(7*10⁻⁴*1.5)=0.0324M and therefore the pH is -log(0.0324)=1.49 ? Or is that wrong since there may exist other H⁺ in the solution that came from water molecules colliding, rather than from the HF?

[Robodexterous]

its not dissassociate!! its dissociate:D

otherwise its amazing!

[jmertelj]

Lol, you make this fun, especially your drawing :D

I probably won't fail the year because of you (:

[FarrahGreye]

lost me at 13:28 but we assume x is very small anyway.... did i say ur awesome!! thank u!!

[GarnetBloodstone13]

i'm a lil confufused isn't HydroFlouric acid the strongest acid as it can burn your skin and fumes are toxic

[MrTrykster]

you totally purified my brain. there is only clarity and hope now.

[qrais]

ugly q in aq sir

[etam89]

To be exact, the Ka is 6.8 x 10^-4

[xfireblasterx]

i think the Ka of HF is 3.5x10^-4

[darrent1000]

10:42 How can I know the number, which is in front of X, is small enough to use your method you show in the box?Can I use your method if the number before the X is 7X10^3 or 7X10^2?

[72fr250]

3 people weren't in aqueous solutions

[tylo38]

i need to find the pH of my citric acid (lemon concentrate) that i titrated with a NaOH base ... i know the pH of citric acid is like 2.3 but i need to show the work...that's where i'm in trouble :l

[chimpi81]

Many Thanx!!! great video

[Cruelendeavours]

Where'd you get the 7 * 10^-4 from?

[faizaan2000]

@Cruelendeavours It is a theoretical value that varies for each acid or base.

[66CrtS]

I smell the Algebra.

[michals000]

good stuff

[narcomanel]

Ahahaha "a proteine, a pro..uh...electron " XD

[ManUtdMU7]

Hey, you know how you did that little approximation thing and got the same answer for both approximation and non approximation number? Yea that is completely legal. My AP Chemistry teacher taught it to us, and its called the "400 rule". When you divide the concentration of acid (or base) with its Ka value (or Kb value for base) and the answer comes out to be greater than 400, then you can ignore the x value in the concentration. So you can ignore the "x" in 1.5-x. :)

[JBilal]

@ManUtdMU7 That's an interesting tip :) Thanks.

[qqqqqppppp5]

can you tell me the reason why you took 10.5 x 10-4 to the .5 power????

[JBilal]

@qqqqqppppp5 Anything to the 1/2 power is the same thing as saying the square root of something. So for example the square root of 4 = sqrt(4) or you can also say 4^1/2 which means the same thing. The 0.5 just represents the square root.

[dirtyrockband]

@qqqqqppppp5 the square root of 10.5x10^-4 is the same as 10.5x10^-4 to the 1/2 power, or .5. Good luck!

[ehab962]

ph for polyprotic acids plzzzzzzzz

[Cesariono]

@ehab962 Consider Sulphuric acid. When 1 mol of Sulphuric acid disassociates, you get one mole of the sulphide ion and TWO MOLES of hydrogen ions so if you have sulphuric acid in an aqueous solution, with a concentration of C, then the Hydrogen ion concentration will simply be double that, 2 * C.

If it was Phosphoric acid then it would be triple and so forth.

[Bythesw23]

you can also see ignoring x as ignoring the low discriminant.

[GabhrooShakeen09]

not a good method. if HF is already known as 1.5 you know the products would have a molarity of 1.5 because of the mole ratio.

[rubixcubemister]

 /watch?v=dEdEJ2X6m4E

[Absinthus]

Greaat videos.

[Thegeaza]

to complicated ive got an easier way of doing this.......

[CrocodileCookies]

Thank you! This is an excellent video for summer time studying ^____^

[GuardianZephyr]

why is 4 times x2 = 1?

[Anikaangellove]

I make that mistake all the time.

Fl is not fluorine, have to slap myself everytime ;)

[dnordon]

Sal, I'm half way through your Chemistry playlist and I just had to say that even though I did chemistry at Sydney Uni ~30 years ago there were many concepts I just didn't really get. Your playlist is making the whole set of concepts behind chemistry much clearer to. This current video on acids is a good example, I kind of knew that acids were but your explanations have given me a much more fundamental understanding I'm watching the videos because I enjoy them - I can't thank you enough

[DyingElement]

HFI, Hydrogen Fuel Enhancement

[Eagles0818]

I am so getting a 5 on my AP Chem exam :D

Thanks!

[jessssss624]

wait... what about the 5% rule? shouldn't x be disregarded?

[jessssss624]

where can i get a chart so i can look up Ka values?

[icare4it]

Hey Sal, can you please use the CASIO CFX-9850gc plus calculator when u calculate? it is difficult to understand design that uses like x ^ y and 0.5 than =

anyway thank you

[dayravager]

This may be nitpicky, but the better term for HCl is ionization, not dissociation.

Anyways, nice vids :)

[bestlessonslearned]

You are Wonderful Khan! Can't thank you enough for teaching all of us....You are truly appreciated!

[MrHibye23]

Fluorine is F, not Fl

[ortizfl1]

Isn't Fluorine F and Fluoride F-?

Why is Fluoric Acid HFl instead of HF?

[febbra2]

YOU.... ARE..... AWESOME!!!! : )

[watergunna]

How do you find the Ka without looking it up, but by calculation?

[wally25feb]

there is a chart for k value . you are given that in the exam .

[khawks123]

so u probably just saved my college career!

[Gotenks56]

@ 1:58 You said that Fl- is a cation, when it's a anion.

Hope you had a note and I'm sorry if I understood it the wrong way

[VanessAssenaV]

I dont understand how and where you got the 7 x 10^-4?? Can anyone help me?

[Gotenks56]

It's a constant for the Kahf @ 25ºC

[marbdy]

@VanessAssenaV

I'm pretty sure you gotta look it up.

[Manu9324]

oh man?

gr8 explanation

[cow2001]

One of the explanations I've heard for how acidity increases as you go down the halogen column is that the extra electron's orbital gets bigger and because of that the charge is being dissipated, making it much stabler for I- than F-.

Have you ever thought on using vector animation app instead of using screen capturing app and a raster imaging app? It'll make it much easier to re-edit later, wouldn't it?

[vickiormindyb]

Trust most scientists to have eagle eyes. The one I lived with noticed my eyes had once been operated on when he was supposed to be gazing into them in a romantic way. That sort of ruined the mood.

He couldn't help himself. I think they train it into them in school.

[gamefaq]

just a small error. fluorine is F not Fl. but overall it was a very informative video. great job like always!

[khanacademy]

thanks for pointing that out. I added an annotation.

[chrisisbest04]

why are you such a genious LOL

[GabhrooShakeen09]

@khanacademy whats fl mate???

anyways good videos khan

[GabhrooShakeen09]

@khanacademy you just showed HF disassociating completely anyways??

so it is a strong acid. lol

[upabittoolate]

BEST vids on youtube.