Chemistry - Equilibrium - Ionization Constants

[Tetra]

We're currently on the Equilibrium unit, and I'm having problems with some questions.

 

1) Given a 0.015 M solution of HOCN with a pH of 2.4, find the ionization constant, Ka.

 

I know that ka = ([H+] [OCN])/ [HOCN], and i can calculate for the concentration of H+ and HOCN. But how do we find the concentration of OCN>?

 

 

2) Given a 0.015M solution of an unknown base with pH10, find:

a) The concentrations of hydroxide and hydronium ion

b) Kb

 

I think for part a) I use the -log[H+] = 10 to find the concentration of H+. But then what about hydroxide? And as for b),

 

Kb = Kw / Ka, but since we don't know Ka, how do we solve for this?

 

Sorry if it's a bit confusing, but I'm also confused. I really don't understand this unit.

[John Cuthber]

Do you know what Kw means?

When you add OH- ions to water what happens to the H+ ions?

[Tetra]

Kw is the equilibrium constant of water...but I'm not sure what that means.

 

As for adding OH, it would react with the H to form water molecules?