posted by Anonymous .
Sodium Carbonate is said to be a strong base however when in aqueous solution it doesn't yield OH^- ions so how can we say its a base.
Or do we say its a base because of its PH.
The solution does yeild OH- ions as it is the salt of a strong base and weak acid.
Consider the two equilibria
H2O <-> H+ + OH-...........(1)
H2CO3 <-> 2H+ + (CO3)2-....(2)
when Na2CO3 is added to water it is a salt and totally dissociates to Na+ and (CO3)2- ions.
The carbonate ions then immediately set up the equilibrium in equation (2). This equation lies to the left as I have drawn it, i.e. it is a weak acid and not totally dissociated. So in order to form th un-ionised acid it must remove H+ from the equilibrium in equation (1), and in doing so forces the equilibrium in equation (1) to the right to produce more OH-. Hence the solution contains OH- ions and is highly alkaline.
"Sodium carbonate is said to be a strong base" in the sense that the salt is totally dissociated.