Question

Which ones are weak and which are strong?

Answer 1

Explanation:

Strong bases are bases that virtually completely dissociate in an aqueous solution.
Weak bases are bases that only somewhat dissociate.

• `Sr(OH)_2`

We can see that the base is `Sr(OH)_2`, and that it basically completely dissociated when placed into an aqueous solution.
So, we can safely say that this is a strong base.

• `NH_3`

Our original base is `NH_3`, and we can see that, while some dissociated in water to form `OH^-` and `NH_4^+` ions, many still stay as `NH_3`.

`NH_3(aq) + H_2O(l) rightleftharpoons NH_4^+(aq) + OH^(-)(aq)`

This means that it does not completely dissociate in water, therefore making it a weak base.

• `NaOH`

`NaOH` dissociates basically completely in an aqueous solution to form `Na^+` and `OH^-` ions, just like `Sr(OH)_2`, so it's a strong base.

• `NaHCO_3`

This image is a bit more difficult to understand, because there are actually two reactions taking place here.

The first reaction is the dissociation of `NaHCO_3` into `Na^+` ions and `HCO_3^-` ions.

`NaHCO_3(aq) -> Na^+(aq) + HCO_3^(-)(aq)`

This happens with basically every `NaHCO_3` compound, but that doesn't make `NaHCO_3` a strong base because, while it virtually completely dissociates, it's not a base.

Then, some `HCO_3^-` ions accept a proton from `H_2O` molecules to form `H_2CO_3` and `OH^-`:

`HCO_3^(-)(aq) + H_2O(l) rightleftharpoons H_2CO_3(aq) + OH^(-)(aq)`

But we can see that this reaction doesn't happen too often, because many are still staying as `HCO_3^-`.
Therefore, this would be a weak base.