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Salt Hydrolysis equation for zinc(II) chloride added to water

Submitted by natash on Sat, 03/14/2009 - 22:20

Okay I cant seem to figure out how to write the hydrolysis equation for .10 M ZnCl2...this is a salt, are you supposed to first figure out if its acidic or basic ? I think this is pH neutral ??? What would the estimated Ka or Kb be for this ? Also for .10 M KAl(SO4)2 I'm lost on this one...what would the Ka or Kb be for this ?


The pH of a salt depends on the type of acid/base used to make it. Salts of strong acids/strong bases are neutral in water;salts of strong acids/weak bases are acidic in water;salts of weak acids/strong bases are basic in solution. Salts of weak acids/weak bases are more difficult to assess as it depends on their relative weakness. Acids such as HCl, H2SO4,HNO3 are strong (there are some others), acids such as organic acids are weak. Bases such as NaOH, KOH, LiOH (and some others) are strong. Bases such as NH3 solution (NH4OH), amines, Al(OH)3, Zn(OH)2 are weak. If you have Ka/Kb values, this will tell you the 'strength' as the lower the value, the weaker the acid/base.

ZnCl2 is the salt of a weak base (Zn(OH)2) and strong acid (HCl). In water it will be acidic. The reason for this is the way that the ions of the salt, Zn2+ and 2Cl- interact with the equilibrium in water, H2O H+  +  OH-
If any ion from the salt can attach to, and remove one of the ions in this equilibrium, the equilibrium will shift to oppose that change. There is no chance of Cl- ions removing H+ ions from the equilibrium as H+Cl- is a strong acid which exists as separate ions in solution. However, Zn2+ ion remove OH- ions from the equilibrium
Zn2+(aq)  +  2OH-(aq)  Zn2+(OH-)2
This is also not very soluble so this equlibrium is to the right. This reduces the concentration of OH- in water, and more water ionises to try to restore the balance (Le Chatelier). As the salt provides a lot of Zn2+ ions, significant numbers of OH- ions are removed from the water equilibrium and the concentration of H+ ions increases - the pH drops (becomes acidic)
With the other salt, one of the two bases (KOH, Al(OH)3) used to make it is weak and I suspect it will also be acidic in water. If you have Ka/Kb values for the acids/bases check this out.
I'm sure there is more than enough info here to set you going.

Have a look at http://www.chalkbored.com/lessons/chemistry-12/pH-salts-handout.pdf
It will help