Question about Kw (solved)

[lex1489]

Advertisement - Members don't see this ad

Just came upon a destroyer question that made me question my understanding of Kw.

If Kw = 5 x 10^-14, what is the pH?

To solve, use
Kw = [H3O+][OH-]
Kw = x^2
x = 2.2 x 10^-7 = H3O+
pH = 6.65

----------------------
I just googled Kw and answered my own question 😛. I'll continue though because I think it'll be helpful for some people.
----------------------
My confusion stemmed from the answer to that question-- Why does the pH drop just because Kw is larger? If H+ and OH- are equal, shouldn't the solution be neutral?

Answer (from http://www.chemguide.co.uk/physical/acidbaseeqia/kw.html): The solution IS neutral.

[FONT=Helvetica, Arial]A word of warning!
If the pH falls as temperature increases, does this mean that water becomes more acidic at higher temperatures? NO!
A solution is acidic if there is an excess of hydrogen ions over hydroxide ions. In the case of pure water, there are always the same number of hydrogen ions and hydroxide ions. That means that the water remains neutral - even if its pH changes.
The problem is that we are all so familiar with 7 being the pH of pure water, that anything else feels really strange. Remember that you calculate the neutral value of pH from Kw. If that changes, then the neutral value for pH changes as well.
At 100°C, the pH of pure water is 6.14. That is the neutral point on the pH scale at this higher temperature. A solution with a pH of 7 at this temperature is slightly alkaline because its pH is a bit higher than the neutral value of 6.14.
Similarly, you can argue that a solution with a pH of 7 at 0°C is slightly acidic, because its pH is a bit lower than the neutral value of 7.47 at this temperature.
.