Is is possible to have a pH of -1 ????????

[Toothguy80]

Ok, Kaplan is making me angry.
It's full length tests have so many mistakes and they bring my score down.

Anyways, there is this question on the fourth full length exam that says,

which ones are possible pH for an acid?
a) -1
b) 2
c) 7
d) 13

I said it was "b" and "c"
but the answer was "a" and "b" only.
Isn't water amphoteric? it can act as an acid and base, we never had a pH of -1, is Kaplan crazy?

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[Lonely Sol]

Ok, Kaplan is making me angry.
It's full length tests have so many mistakes and they bring my score down.

Anyways, there is this question on the fourth full length exam that says,

which ones are possible pH for an acid?
a) -1
b) 2
c) 7
d) 13

I said it was "b" and "c"
but the answer was "a" and "b" only.
Isn't water amphoteric? it can act as an acid and base, we never had a pH of -1, is Kaplan crazy?

Yea pH can be -1, concentrated HCl have a pH of -1. And the question only asks about the pH, yea water is amphoteric, but that doesnt mean that since there is 7, they are talking about water.

 

[tresbon]

Yea pH can be -1, concentrated HCl have a pH of -1. And the question only asks about the pH, yea water is amphoteric, but that doesnt mean that since there is 7, they are talking about water.

That's right. And a 10 M HCL will render a pH of -1.

 

[joonkimdds]

Ok, Kaplan is making me angry.
It's full length tests have so many mistakes and they bring my score down.

Anyways, there is this question on the fourth full length exam that says,

which ones are possible pH for an acid?
a) -1
b) 2
c) 7
d) 13

I said it was "b" and "c"
but the answer was "a" and "b" only.
Isn't water amphoteric? it can act as an acid and base, we never had a pH of -1, is Kaplan crazy?

7 is not acid, it's neutral.
the only option u have is a and b but i don't know if it's possible to be negative.

 

[nicko18]

7 is not acid, it's neutral.
the only option u have is a and b but i don't know if it's possible to be negative.

If you knew how pH was worked out you'd realise it certainly can be negative.

pH is simply the -log[H+] ions.

Having a solution with 1x10^-7 [H+] gives you a pH of 7
Having a solution with 1x10^1 [H+] given you a pH of -1

you can clearly see that the more [H+], the lower the pH is going to be.

10M HCl has a pH of -1

 

[nicko18]

Ok, Kaplan is making me angry.
It's full length tests have so many mistakes and they bring my score down.

lol - i warn those who keep getting low scores to question their own answers before that of Kaplan 😉

 

[nicko18]

Isn't water amphoteric? it can act as an acid and base, we never had a pH of -1, is Kaplan crazy?

ermm.. no.

An acid is defined as a substance that increases the number of H+ ions when dissolved in water. Something with a pH of 7, including water, will not do that.

But yes, water can 'act' like an acid or a base depending on what you mix it with, it's going to create more H+ or OH-, but that isnt what defines an acid.

 

[JMJRDH1]

ermm.. no.

An acid is defined as a substance that increases the number of H+ ions when dissolved in water. Something with a pH of 7, including water, will not do that.

But yes, water can 'act' like an acid or a base depending on what you mix it with, it's going to create more H+ or OH-, but that isnt what defines an acid.

Actually, it depends on which definition of an acid or base you are using.

Your example is of an Arrhenius acid, which releases H+ ions in solution.

A Bronsted-Lowry acid is an H+ donor, and a base is an H+ acceptor.

A Lewis acid is an electron pair acceptor, and base is an electron pair donor

 

[joonkimdds]

If you knew how pH was worked out you'd realise it certainly can be negative.

pH is simply the -log[H+] ions.

Having a solution with 1x10^-7 [H+] gives you a pH of 7
Having a solution with 1x10^1 [H+] given you a pH of -1

you can clearly see that the more [H+], the lower the pH is going to be.

10M HCl has a pH of -1

then is there any reason why bio, chem textbook gives a range between 1 to 14 for acid and base?

 

[jackbauer!]

then is there any reason why bio, chem textbook gives a range between 1 to 14 for acid and base?

for all realistic purposes, most working acids and bases will have a pH between 0 and 14. Those acids that have a pH below 0 are generally stock acids and need to be diluted to be used for any experimental purpose.

Hope this makes sense.

jb!🙂

 

[zoom12]

Even though pH = -log(H+) is more or less true, it doesn't always work by just plugging in the molarity of the acid,

For any pH measurements, the only time you can consider the proton contribution from the added acid alone is if the concentration is a few orders of magnitude greater than the concentration of H+ from water . This is because when your acid starts becoming this dilute you need to consider the autoprotolysis of water (i.e. water's inate tendency to lose protons to the solution just like any other acid - anything with protons has a dissociation constant). For all intents and purposes for most acids, the protons from the strong acids are so much greater than that of water, we just ignore the H+ contribution from water (ka = ~1.6*10^-16).

So when your acid becomes that dilute, water becomes the stronger acid (oddly enough, water actually is an acid since it has an acid dissociation constant). If it didn't, and you had a 1*10^-8 M HCl solution the pH would = 8 (which cannot be true since HCl isn't a base)

Hope this helps!