# GC Destroyer 2013 #228

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#### swolly

##### Full Member
7+ Year Member

Alright guys.. so question is at 37 deg Celcius, Kw for H2O is 5 x 10 ^-14, what is Ph at this temperature.

So i solved and got the correct answer by setting the Kw = (OH-)(H+)... then going to 5x10^-14 = x^2. Then i proceeded to solve for x and got ~2.5 x 1o^-7.. plugged this into pH= -log (H3O+) and got the pH is equal to around 6.7 or so... ok so thats all good.

But wouldn't that also mean that i could have used the same value of x (2.5 x 10^-7) for (OH-) and used that in pOH = -log(OH-)?? but then I would get the same pOH as pH and that doesn't make sense... I feel like I am missing something here? Could anyone explain?

#### Project Pat

##### Full Member
5+ Year Member
You need to remember that for any given temperature, [H+] = [OH-] for water, so water is neutral at all temperatures. So you are correct in saying that you could use the same number to calculate pOH. Also remember that pH + pOH = 14 is only true at 25oC

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#### elhammy

##### Full Member
5+ Year Member
Alright guys.. so question is at 37 deg Celcius, Kw for H2O is 5 x 10 ^-14, what is Ph at this temperature.

So i solved and got the correct answer by setting the Kw = (OH-)(H+)... then going to 5x10^-14 = x^2. Then i proceeded to solve for x and got ~2.5 x 1o^-7.. plugged this into pH= -log (H3O+) and got the pH is equal to around 6.7 or so... ok so thats all good.

But wouldn't that also mean that i could have used the same value of x (2.5 x 10^-7) for (OH-) and used that in pOH = -log(OH-)?? but then I would get the same pOH as pH and that doesn't make sense... I feel like I am missing something here? Could anyone explain?

Kw is used for PURE WATER. Pure water only dissociates into Hydrogen ions and Hydroxide ions. You are merely calculating the new pH. pOH should equal pH in a pure water liquid suspension. What you are saying is as you increase the heat of water, you should have a more acidic solution (since pH is going to be <7) so you assume that water has more H+ ions. This is incorrect because temperature changes only affect Kw but not pH only for pure water. You basically calculated the neutral pH at that particular temperature. Anything higher would be more alkaline and anything lower is more acidic (if you add any substances to it that ionize).

When i first read this, I was freaking out lol. You made me doubt myself. But water is the only substance that retains its neutrality on a pH scale regardless of its pH or pOH.

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#### swolly

##### Full Member
7+ Year Member
lol wow this is getting pretty deep... ok.. so pretty much the H+ concentration does increase by the higher temp (37) which therefore decreases the pH, but it's still neutral because the same amount of OH- dissociates as well... so the pOH is equally decreased. So in this scenario pH + pOH does NOT equal 14 since kw is slightly higher than 1x10^-14... so it does make sense that pOH = pH, but in a problem involving pH in a temperature other than 25 deg C I shouldn't use pH + pOH = 14 and in this specific scenario pH + pOH is prob closer to ~13 or so.. (probably not important just for clarification purposes)?

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#### Daneosaurus

##### D4
7+ Year Member
Guarantee you won't see this on the DAT

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