AQA A Level Chem Reactions of aqueous ions

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For question 5.2 How do you know that NaOH is excess OH- ions?
And that the Formula of C is [Al(OH)4]- not [Al(OH0)4(H2O)2]-

Hi there!

NaOH needs to be in excess as otherwise the complex could precipitate as follow when it achieves a neutral charge:

[Al(H20)]3+(aq) + 3OH-(aq) ⇌ [Al(OH)3(H20)3](s) + 3H2O(l)

Regarding the [Al(OH)4]- not [Al(OH0)4(H2O)2]- to me they are both correct as the first one it is just simplified (sometimes the ionic equation using Al(OH)3 is used for example). As there isn't an actual ligand exchange taking place but the high charge density of the cation weakens the O-H bonds of the aqua ligands so H+ ions dissociate when react with OH- ions forming the H2O products and leaving OH- as a ligand instead. Hence the 2 remaining aqua ligands have not been removed as far as i know. It seems to me that there is a bit of leeway about it but don't take me for sure on that. Maybe someone else can help on that.

I hope it all make sense! Please let me know how you get on