Question

How to measure the hydronium ion concentration of water? :)
EDIT: like I know it's 1x10^-7M but how

Answer 1

@ganeshie8 @myininaya @.Sam. @thomaster @timo86m @dan815 @Compassionate @shamil98 @TheViper @FutureMathProfessor @kewlgeek555 @AllTehMaffs

Answer 2

@Vincent-Lyon.Fr Aide-moi si tu peux :)

Answer 3

\[ 2 H_2O \overset{\leftarrow}{\rightarrow} OH- + H_3O^+\]
In pure water there are an equal number of hydroxide ions as hydronium ions, and

pH = 7
pOH = 14 - 7 =

pOH =7 means
\[7=-\log{a_{OH-}}\]
and you're looking for aOH

Answer 4

Measure its pH. If you find pH = 7, then it means the concentration is \(10^{-7}\) mol/L

Answer 5

Top reaction is auto-dissociation in water

Answer 6

But pH comes from the concentration, you can't reverse it...

Answer 7

?

Answer 8

like you know water's pH is 7 because its hydonium ion concentration is 1x10^-7

Answer 9

pH hydrogen ion concentration, and pOH s hydronium ion concentration - in water they're related by pH + pOH = 14

I don't understand what's being reversed however...

Answer 10

how do you know that pH+pOH=14...

Answer 11

It's from an equilibrium constant that governs the above chemical equation (I'm not very well versed in it - I was just always taught that was true)

http://en.wikipedia.org/wiki/PH#pOH

from
\[pH = pK_w - pOH\]
and
\[pK_w \approx 14 \ \text{at room(ish) temperature}\]

Answer 12

ok...

Answer 13

with a pH meter?

Answer 14

I appear to have been incorrect, I'm very sorry - pH seems to effectively be a measure of the concentration of hydronium ions, so since the pH of water is 7, then the concentration of hydronium ions is
\[ [H_3O^+] = 10^{-pH}\]
which was the answer you were looking for. Again, apologies.

Answer 15

Also, above, when I mentioned that pOH was hydronium I mistyped - pOH is hydroxide ions, (OH- ions). That's why everything I said was so confusing I think (also because it was mostly incorrect. So again, apologies).

Answer 16

but I mean how do you know that water's pH is 7 in the first place

Answer 17

I'm not entirely sure how to mathematically derive it - it's just generally what pure water is taken to be. Sorry that it's a cop out answer :P

Answer 18

so the pH of water is defined to be 7?

Answer 19

yes it is, and (in a more rigorous approach) it's based on the activity of hydronium ions, not their concentration.

Answer 20

ic