pH + pOH = 14? I'm confused.

[euphaire]

Hi all,

Can someone tell me why the pH + pOH = 14 and whether or not if this applies in ALL situations. Why or why not.
A question that got me thinking was from this year's Destroyer:
Q: At 37C, Kw for water is 5x10^-14. What's the pH?
A: pH is 6.6 because Kw = x^2 and from x you get a pH of 6.6.
HOWEVER, x also equals to [OH-] (obviously, [H+]=[OH-] here), from which you get a pOH of 6.6. Then assuming pH = 14-pOH, the pH is now 7.4 instead?!

I don't really understand why most of the time pH + pOH = 14 works but sometimes, like in this problem, it doesn't make any sense.

Thank you so much in advance!

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[needzmoar]

Hi all,

Can someone tell me why the pH + pOH = 14 and whether or not if this applies in ALL situations. Why or why not.
A question that got me thinking was from this year's Destroyer:
Q: At 37C, Kw for water is 5x10^-14. What's the pH?
A: pH is 6.6 because Kw = x^2 and from x you get a pH of 6.6.
HOWEVER, x also equals to [OH-] (obviously, [H+]=[OH-] here), from which you get a pOH of 6.6. Then assuming pH = 14-pOH, the pH is now 7.4 instead?!

I don't really understand why most of the time pH + pOH = 14 works but sometimes, like in this problem, it doesn't make any sense.

Thank you so much in advance!

To be clear, pH+pOH=pKw
pKw is only 14 @ 25C which clearly isn't the case here. This question was asked earlier in this thread

http://forums.studentdoctor.net/showthread.php?t=850466

So in this case pKw = -log[5E-14] = 13.3

 

[AlbinoPolarBear]

Hi all,

Can someone tell me why the pH + pOH = 14 and whether or not if this applies in ALL situations. Why or why not.
A question that got me thinking was from this year's Destroyer:
Q: At 37C, Kw for water is 5x10^-14. What's the pH?
A: pH is 6.6 because Kw = x^2 and from x you get a pH of 6.6.
HOWEVER, x also equals to [OH-] (obviously, [H+]=[OH-] here), from which you get a pOH of 6.6. Then assuming pH = 14-pOH, the pH is now 7.4 instead?!

I don't really understand why most of the time pH + pOH = 14 works but sometimes, like in this problem, it doesn't make any sense.

Thank you so much in advance!

Lol, howd u get such crazy grades at UT without knowing this? 😛

 

[Hope30]

I want to point out some points for this kind of pH question and the scientific notation.

5 x 10^-14
Temperature of 37C is one factor you can determine that it is slightly acidic. Another one would be (5) from the notation. As the number gets higher, it is more acidic. Another way you can look at it as 14-log(5), so it is less than 13.

Looking at the question with the given temperature of 37C and scientific notation of 5. You can tell that water of pH 7 is slightly acidic. Any answer choices close to 7 but less than 7 is your answer. I hope this will make thing easier for you.