pH of a Weak Acid

[@prabhanshukatiyar4635]

hydrofluoric acid ain't HFl, that simply HF

[@sperg1]

@Piotr B. He "isn't" wrong though

[@khanacademy]

thanks for pointing that out. I added an annotation.

[@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

[@SharronJenkins]

The chemical symbol for fluorine should be F, not Fl.

[@Peter_1986]

I love how Sal always ends his videos with a confident "see you in the next video". He just knows that "yep, you WILL watch the next video", lol.

[@steveshillingburg]

Sal, your videos are amazing! Why do you always write Fluorine as Fl?

[@indogen-z2872]

lmao

[@JulienNoahDattaFaber-Rod2022o]

I fricking love you! You are literally the only one on the entire internet that explains how to solve the quadratic formula regarding the pH calculations

[@gamefaq]

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

[@princessbre]

Saw the thumbnail and the illustrated Ka value for HF (as many people probably have pointed out, the element fluorine is only represented by F) and I must say... that’s not the Ka constant for equilibrium at 25 degrees for HF. It is truly 3.5x10^-4.

[@eternity9857]

1.5M of hydroflouric acid is already IN equilibrium with an unknown concentration x of H+ and F-. That is why the equation is x^2 = 10.5 ^ -4, not because of this arbituary notion that because x^2 is larger than 10.5^-4x therefore you cab ignore an entire variable.

[@sainikhilchakravarthi9091]

Yeah

[@fluting4ever]

electronegativity, Cl has less shielding so it retains the electrons of Hydrogen more tightly, making it harder to break. Br has more shells, so the bonding is weaker, easier to break.

[@aymansays]

lol at "i think you can smell some algebra coming our way" at 8:33

[@MrFarkasOfficial]

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

[@spikygoldfish]

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

[@jennifercarmona9788]

update??? did you???

[@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.

[@JohnSmith-nq1zg]

lol it's HF not HFl 1:40 Floride is just the F bro great vid anyway though :)

[@JohnSmith-nq1zg]

my bad he corrected it later in the vid keep up the vids they're saving me in AP chem!

[@bestlessonslearned]

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

[@markhillband7635]

HEY COME ON!!!! Theres like 20 people in the comment section below who dont understand where you got 7 X10^-4 from??????? Can someone please explain???? He said it was a constant...but you gave the formulas then no values then a magical number

[@JackJack-cf7le]

+D.I. L.F It's a constant. On a test or exam they will give it to you OR it will be written on the formula sheet they give you on the AP exam. In the real world you would just Google it.

[@elwynbrooks]

I'm really glad to see that I'm not the only one who accidentally puts fluorine as "Fl" sometimes.

[@salonirao9210]

I think khan academy is gods way of apologizing for AP classes

[@susansong963]

preach ;D

[@faizaan2000]

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

[@imalazykid]

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

[@vpanopoulos]

What I don't understand in this video is at the very end. When you calculate the pH to be 2 (I rounded up) how is that considered to be a weak acid? Because if you ask yourself, "is a pH of 2 indicative of a weak acid?" the answer is no. So how do you account for the inconsistency? I only ask b/c I'm taking the MCAT soon and this is a problem area for me. Your videos have really helped through this tough material.

[@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. :)

[@Lymanity]

That short cut around the quadratic is NIFTY!

[@etam89]

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

[@siveduncan9551]

Just a quick question! Why are the concentrations of H+ and Fl- be the same at equilibrium? Why wouldn't one be x and the other y? And if they are the same then why isn't it 0.15-2X? Any help would be greatly appreciated !!!

[@JPohl-np3hb]

Sive Duncan Every HF that gets disociated is divided into exactly 1 H+ and 1 F-. In other words, every H+ from the disociation has his F- "partner", so their concentrations in the solution are the same: [H+]=[F-]. For the "x and not 2x", just imagine the solution: one part of the HF gets disociated while the other stays the same. Say you put 5 molecules of HF into 1 liter of water, and say 3 of them are disociated. The disociated ones are now separated H+ and F-, which means that the concentration x which is the concentration of one of the two compounds is equal to the whole concentration of the disociated HF.

[@yeeeeeeeeeah]

Thank you! Also does weak acid mean the compound is more stable that its less likely to dissociate hydrogen?

[@agent475816]

You forgot to mention you are assuming things. You are assuming that [H+] from kw is negligible. You are also assuming that the [OH-] from kw is much less than [A-]. So you are saying that [H+] ~ [A-] and not including [OH-]. Which is fine but you didn't mention it.

[@agranavaratne7731]

Yeah shouldn't there actually be a common ion effect on both water and the dissociation of HF

[@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.

[@WhiteRazr1233]

I think you just saved me for my Chem 2 test in the morning

[@FarrahGreye]

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

[@user-xx4vd1yr4m]

Your videos are going to save me for my chem midterm tomorrow! Ive already watched 5 hours worth of videos and i still have another 2 to go. Thanks guys!

[@taylorwsims]

Do you have any videos for calculating pH of a very dilute weak acid or base where the auto-ionization of water is taken into account?

[@melrakan]

I encountered the following thing when solving equations. x^2+7.2*10^-4x-1.8*10^-4 How do I solve this without using a calculator (and that doesn't seem to work either)? I really have no idea.

[@agent475816]

Yea, you have to solve it using the quadratic formula.

[@thebadasses183]

Thank you a lot, finally I understand this kind of questions!

[@shersher1492brewer]

2:46 "They tend to be very willing to give it up to these guys."

[@eneanasto2647]

try speed 2... takes like 10 seconds to get used to but yeah 3 hours left to the exam

[@ibchemvids]

This video needs to be redone - full of errors - gimme a call Sal - I'll put you straight!

[@agranavaratne7731]

Shouldn't there actually be a common ion effect on both water and the dissociation of HF.

[@CrocodileCookies]

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

[@Thresher042]

Keep it up Sal, you're awesome.

[@trailertrashtactics]

Sal.. the master of this.. is struggling figuring out this problem with a calculator and i'm suppose to find it out without one? oh boy

[@indian310]

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

[@secretiveInfidel]

Why do you need to use a Quadratic equation? u just need to get the square root of 10.5x10^-4 ( 7x10^-4) it will be equal to x. And the answer is 2.27x10^-5 the pH is 4.64... That pH is more convincing as a Weak acid than your pH of 1.9 :| Im Just saying what I've computed. Tell me if I'm wrong

[@knitwithkayla]

Sal, do you use the "ICE" method to solve for concentrations? I just find that easier to visualize, and I was wondering which method works better? Or are they the same?

[@upabittoolate]

BEST vids on youtube.

[@qqqqqppppp5]

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

[@源氏-e8i]

Question, where did ylu get the 7x10^-4 ? And why did the 7x10^-4 remained even it was multiply by 1.5 ? Sorry i didnt understand that much sorry

[@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?

[@skateandtennis]

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

[@Cruelendeavours]

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

[@eduardoschiavon5652]

That's just the value of the constant, he probably got it from an equilibrium constant table.

[@mayko2468]

That's something that has already been calculated like the electronegativity , or just like water which has a K equal to 10^-14

[@CurrentlyDisassociating]

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

[@khawks123]

so u probably just saved my college career!

[@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.

[@CurrentlyDisassociating]

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

[@ibchemvids]

Why not redo this vid and fix the errors?

[@sarahh.1716]

So for any weak acid, Ka = 7.0 * 10^-4 ?

[@hussnain6114]

No

[@jmertelj]

Lol, you make this fun, especially your drawing :D I probably won't fail the year because of you (:

[@Bizkid10]

How come you didn't just use ICE? it woulda been easier to show with X and Y

[@VanessAssenaV]

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

[@kulunuabeysinghe7424]

KZbin is my school and you are my class teacher

[@elsjehinsbeeck1199]

In exams and stuff, will the Ka value ne given?

[@t.c.5456]

No matter how you put it, this guy is the best to do it. Best who ever did in (online anyways)

[@agent475816]

Everybody does it like this.

[@BeybladeFables]

Learn more from these videos than you do in lecture

[@ina_5832]

Or you can make it [H+]=square root of Ka * Ca

[@SuperMaester]

5:40 then why is hydrochloric acid stronger than hydrobromic acid

[@abdurahmanjeego7068]

Thank you salkhan.

[@tvlersalinas]

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

[@agent475816]

Flurine is F not Fl haha

[@gagan4012]

F is Flourine not Flurine haha....The point that im trying to make is mistakes happen..

[@daniloboskovic9671]

not to someone who actually has a degree

[@Peter_1986]

@@daniloboskovic9671 People with a degree make mistakes as well, just like everyone else. You don't become some flawless robot without flaws just because you have a degree in something - engineers are humans as well.

[@bradyirvine895]

My chemistry teacher ate this video alive... "Within the first two minutes Khan Academy made 6 mistakes. It somewhat okay, but don't review from it, you'll fail."

[@Origination1]

Isn't Flerovium written as Fl?

[@GuardianZephyr]

why is 4 times x2 = 1?

[@we2mod]

How did you get 7x10^-4 !?

[@muftahf.s7535]

we2mod These is costant ... any acid has constant ka.. in mostly you are found in problem " example"

[@nimageemon9520]

HFl is hydrofluoric acid??

[@JBilal]

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

[@alivingwill1]

One does not simply.... make a box.

[@TheJHrevolution]

2:54 the guy laughs at himself for saying "gives up PROTEINS"

[@southpark1029384756]

Thank you!

[@seemoremacstuff]

Thank you

[@MG-xn5uw]

this video very helped

[@Justinmi010]

i thought this video was calculating the ph of a WEAK acid... is a pH of 1.5 considered a weak acid?

[@kkirlin07]

no, weak means pH close to 7, 1.5 is the molarity of the acid not the ph, pH closer to 7 the acid-base weaker. a weak acid does mean the pH value is close to neutral because not 100% of the H ions will dissociate. a weak acid with a pH of 1.5 is impossible, and 1.5 is not a high concentration. are u kidding me???? most of what u said are wrong

[@trinhsanity8484]

Love hearing dads voice i mean dads voice i mean sals voice

[@lazypunk794]

he keeps sayin "diassociated " for dissociated.

[@michals000]

good stuff

[@febbra2]

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

[@favoritechannel783]

why ph is between 0 to 14

[@chrisisbest04]

why are you such a genious LOL

[@ChaosLordQuackers]

one does not simply... write (aq) in green.

[@janm9960]

I don't understand that

[@willow1279]

i'm f*$%£"! confused with this square roots and all that.... :(

[@sciencenerd7639]

Oh whoops he wrote Fl instead of F again! That's okay I still love Khan Academy anyway.

[@ShahrukHossain]

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

[@Absinthus]

Greaat videos.

[@hamzawasim5218]

This is really good but you ramble too much

[@Sigmath_Bits]

I smell the Algebra.

[@ShahrukHossain]

@imalazykid thanks i realized later on :D

[@72fr250]

3 people weren't in aqueous solutions

[@dedly13]

i think he notices in a later video

[@DragonIgnil]

HFl haha, always funny when "instructors" make mistakes :) Very helpful video.