Can a weak acid ever have a pH of less than 0? Watch

outrun
Badges: 2
Rep:
?
#1
Report Thread starter 7 years ago
#1
Can it ever have a pKa of less than 0?
0
reply
EierVonSatan
Badges: 19
#2
Report 7 years ago
#2
pH or pKa?
reply
No Future
Badges: 15
Rep:
?
#3
Report 7 years ago
#3
Wouldn't a weak acid be closer to pH 7?
0
reply
foianorsx205
Badges: 0
Rep:
?
#4
Report 7 years ago
#4
No... for the pH that is.
0
reply
andyator55
Badges: 0
Rep:
?
#5
Report 7 years ago
#5
If this isfor PH then its impossible to get a PH lower than 0
0
reply
chemdweeb1234
Badges: 0
Rep:
?
#6
Report 7 years ago
#6
(Original post by andyator55)
If this isfor PH then its impossible to get a PH lower than 0
If i recall correctly, some STRONG acids can indeed have a negative pH but in the case of WEAK acids the question is out of having a pH close to 0....
0
reply
Coke Or Pepsi
Badges: 2
Rep:
?
#7
Report 7 years ago
#7
(Original post by chemdweeb1234)
If i recall correctly, some STRONG acids can indeed have a negative pH but in the case of WEAK acids the question is out of having a pH close to 0....
Yep. You can have a pH that is negative. -Log2 for example gives you a negative number.

What is the point of neg repping? Really lol.
2
reply
User570431
Badges: 15
#8
Report 7 years ago
#8
Yeah a PH debate ....And I thought I was a science nerd :rofl:!
Edit neg rep? I'm just proving a point...
reply
sicarius1992
Badges: 11
Rep:
?
#9
Report 7 years ago
#9
(Original post by blueray)
Yeah a PH debate ....And I thought I was a science nerd :rofl:!
Well it is a chemistry forum darling.
2
reply
User570431
Badges: 15
#10
Report 7 years ago
#10
(Original post by sicarius1992)
Well it is a chemistry forum darling.
You do realise i'm a male and so are you.
reply
andyator55
Badges: 0
Rep:
?
#11
Report 7 years ago
#11
(Original post by chemdweeb1234)
If i recall correctly, some STRONG acids can indeed have a negative pH but in the case of WEAK acids the question is out of having a pH close to 0....
Yes I meant what I said in relation to the question. Sorry if anyone misunderstood what I said.
0
reply
shaz111
Badges: 1
Rep:
?
#12
Report 7 years ago
#12
no i dont think so, its something to do with weak acids not ionisng completely.
0
reply
soutioirsim
Badges: 13
Rep:
?
#13
Report 7 years ago
#13
(Original post by blueray)
Yeah a PH debate ....And I thought I was a science nerd :rofl:!
Edit neg rep? I'm just proving a point...
The "P", should be lower case...

Just sayin'
3
reply
TomInce6666
Badges: 10
Rep:
?
#14
Report 7 years ago
#14
(Original post by blueray)
You do realise i'm a male and so are you.
And what darling? Object to a bit of man on man affection now?
0
reply
JayTeeKay
Badges: 12
Rep:
?
#15
Report 7 years ago
#15
(Original post by andyator55)
If this isfor PH then its impossible to get a PH lower than 0
I take it you're not at A2 chemistry yet. It's a shame they teach you this so late on.

PH is defined as -log[H+], where [H+] is the hydrogen ion concentration in moles per cubic decimetre.
I'm not sure if you're familiar with logarithms, but PH = -log[H+] means that
10^-PH = H+.
From the laws of logs, PH also equals log[H+]^-1 or log(1/[H+]).

Therefore, if the hydrogen ion concentration is greater than 1, the term inside the brackets will be less than 1. The only way that 10^PH will ever equal a number less than 1 (1/[H+]) is if the value of PH is negative.
It is therefore possible to have a PH lower than 0 if the hydrogen ion concentration is greater than 1 moldm^-3.

In terms of a weak acid,[ H+] = ((Ka)[Acid])^0.5
In order for [H+] to be greater than 1 (so the PH is negative) one of two things must be true:
1) [Acid] is very large. The acid would have to be impossibly concentrated though, given the typically small values of the acid constant: http://chemistry.about.com/od/charts...a/pkatable.htm
2) Ka is large. For Ka to be large though, the weak acid equilibrium would have to lie heavily to the right so that ([H+]^2)/[Acid] was large enough. This does occur, but only in strong acids, where you can more or less assume that the acid completely dissociates into ions (so [H+] is very high). Weak acids don't dissociate to anywhere near the same extent, hence they are weak acids.

So, in answer to the OP's question, no. A weak acid can never have a negative PH. Only a strong acid can have a negative PH.
3
reply
andyator55
Badges: 0
Rep:
?
#16
Report 7 years ago
#16
(Original post by JayTeeKay)
I take it you're not at A2 chemistry yet. It's a shame they teach you this so late on.
Thanks for that. Everything makes sense now. Not sarcastic by the way. And no just finishing the AS.
0
reply
JayTeeKay
Badges: 12
Rep:
?
#17
Report 7 years ago
#17
(Original post by andyator55)
Thanks for that. Everything makes sense now. Not sarcastic by the way. And no just finishing the AS.
No problem. I myself only covered it a few months ago. You'll learn about equilibrium constants, acid constants and PH values in the second module next year. The second bit relating to the OP's question will make a lot more sense to you then!!
You have so much "fun" stuff to look forward to
Although nowhere near as "fun" as A2 physics by a long shot!
0
reply
megadeath999
Badges: 7
Rep:
?
#18
Report 7 years ago
#18
You cannot get a weak acid with a pH below 0. Strength of an acid is determined by the H+ concentration of the aqueous acid, and in a weak acid you won't get a high enough H+ because the acid won't dissociate fully into H+ and it's conjugate base
0
reply
Coke Or Pepsi
Badges: 2
Rep:
?
#19
Report 7 years ago
#19
(Original post by JayTeeKay)
No problem. I myself only covered it a few months ago. You'll learn about equilibrium constants, acid constants and PH values in the second module next year. The second bit relating to the OP's question will make a lot more sense to you then!!
You have so much "fun" stuff to look forward to
Although nowhere near as "fun" as A2 physics by a long shot!
A2 physics is soooo fun

:////
0
reply
JayTeeKay
Badges: 12
Rep:
?
#20
Report 7 years ago
#20
(Original post by Coke Or Pepsi)
A2 physics is soooo fun

:////
It sure is. It's so fun that I've applied to do another four years of physics at uni!

Maybe I should just shoot myself now...
1
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • Cranfield University
    Cranfield Forensic MSc Programme Open Day Postgraduate
    Thu, 25 Apr '19
  • University of the Arts London
    Open day: MA Footwear and MA Fashion Artefact Postgraduate
    Thu, 25 Apr '19
  • Cardiff Metropolitan University
    Undergraduate Open Day - Llandaff Campus Undergraduate
    Sat, 27 Apr '19

Have you registered to vote?

Yes! (301)
37.48%
No - but I will (60)
7.47%
No - I don't want to (60)
7.47%
No - I can't vote (<18, not in UK, etc) (382)
47.57%

Watched Threads

View All