SA + SB always give pH= 7 even if concentration of SA is a lot bigger than SB?

[cmkc109]

SA + SB always give pH= 7 even if concentration of SA is a lot bigger than SB? let's say 10 M of HCl react with 0.1 M of NaOH, is the pH at equivalence point also 7?

 

[Yanick]

When titrating strong acids/bases (assumed fully ionized) you can just think about the moles instead of the molarity. This is where, it seems, you are being confused. You can titrate a 10M strong acid solution with 0.1mM strong base, you'll just need a lot of the base (because it is much more dilute you'll need to add a lot of the basic solution to match the moles of acid). The equivalence point however, will be at pH 7 at 25 C, because at that point the acid and base number of moles are equal and its as though you dropped in the salt of the counter ions into water.

It may be useful for you to calculate, for instance, how much of a 0.1mM NaOH solution is needed to titrate 25mL of a 10M HCl solution to the equivalence point. Then think about what you will have at the end. If the [H+] = [OH-], then you have pretty much water (and the scant amount of acid/base from auto ionization) and NaCl. It's as though you just made an NaCl solution in pure water.

 

[cmkc109]

Thanks Yanick for clearing my doubt, very helpful :)