Question

How to predict whether the aqueous solution for the following salts are acidic , basic or neutral (a)NH4Br (b)KClO4 (c)NaHC2O4 (d)CsBr (e)KHSO4 (f)[CH3NH3]Cl?

Answer 1

`a. acid; b. "neutral"; c. acid; d. "neutral"; e. acid; f. acid`

Explanation:

You have weak acids, ammonium bromide, sodium oxalate, potassium bisulfate, and ammonium bromide. And neutral salts: potassium perchlorate, and caesium bromide. The only potential problem is sodium oxalate.

The parent acid, oxalic acid, has `pK_(a1) =1.25` and `pK_(a2) =4.14`. Of course, `pK_(a2)` is the one that concerns us. Compare `pK_(a2)` to `pK_a` ammonium bromide, `4.76` (that's from memory, I could not find my old notes, so check on me!). So sodium oxalate is a stronger acid than ammonium bromide, and will thus give also a slightly acidic solution in water.

Another way to look at this is by the strong acid/weak base formalism. Since we are chemists, physical scientists (hopefully!), I would always prefer to get the numbers, and find the `pK_a's`.