[Imo]

why the pka of water is 15 ? i have no idea of this...

[Nescafe]

why the pka of water is 15 ? i have no idea of this...

Start by figuring out the molarity of 1L of water. Then try and calculate the ka in order to figure out the pka of water.

Cheers,

Nescafé.

[Imo]

Then [H+] * [OH-] / [H2O] = 10^-14/55

But, why H2O is in the equation of the Ka?   Ka= [H+][OH-]/[H2O]
should not be :  Ka= [H+][OH-]?

[Borek]

Compare http://www.chembuddy.com/?left=pH-calculation&right=water-ion-product

[Nescafe]

Then [H+] * [OH-] / [H2O] = 10^-14/55

But, why H2O is in the equation of the Ka?   Ka= [H+][OH-]/[H2O]
should not be :  Ka= [H+][OH-]?

Because there is an equilibrium and in order to calculate the rate constant you must account for the concentration of water as well as the ions. I'm on my phone so I'm not sure how this is going to turn out.

H2O <---> H+ + OH-

Water dissociate to H+ and OH-. The dissociation constant would be equal to concentration of water over the ions and ka would be the vice versa. For instance ka would be:

Ka = [H+][OH-] / [H2O]

Cheers,

Nescafé.

[Nescafe]

Then [H+] * [OH-] / [H2O] = 10^-14/55

But, why H2O is in the equation of the Ka?   Ka= [H+][OH-]/[H2O]
should not be :  Ka= [H+][OH-]?

What you are writing is not completely incorrect but it is rather in accurate. A lot of people assume the concentration of water to me 1.0M, however this is not always the case. Of you do assume it to be 1M then the pka would be 14. However the more accurate way is to calculate the molarity of water as you did (55) and use it in your calculation for an accurate calculation of the pka of water.

Cheers,

Nescafé.

[Imo]

And how i figure out the pka of H3O+? is -1,74 ...

[Nescafe]

And how i figure out the pka of H3O+? is -1,74 ...

H3O+ <------> H+ + H2O

The Ka for H3O+ would be defined as Ka = [H+] [H2O] / [H3O+] . [H+] is the same as [H3O+] , so Ka = [H2O] = 55.5 , therefore pKa = -1.7.

Cheers,


Nescafé.