How do I know which of these .1M solutions will have a pH less than 7?
KF, NH4I, NaH2PO4, Fe(NO3)3
I am guessing all but KF is acidic...but if someone could confirm this and explain why that would be helpful
KF will be basic.
NaH2PO4 will be basic.
The other two will be acidic.
But the way I tell is FAR from the accepted way in class rooms. But you are welcome to the method I use.
Write the hydrolysis equation.
KF + HOH ==> HF + KOH
Weak acid, strong base, basic
NH4I + HOH ==> NH4OH + HI
weak base, strong acid, acidic
Technically, it has been shown that NH4OH doesn't exist as such but we taught for years that a solution of NH3 produced NH4OH with Kb = 1.8 x 10^-5 and worked all of our problems that way. The NH4OH intermediate ionized to NH4^+ + OH^-. Now the reaction is described as
NH3(g) + HOH ==> NH4^+ + OH^-. The only real difference is that the intermediate is omitted from the reaction.
Fe(NO3)3 + HOH ==> Fe(OH)3 + HNO3
not balanced.
weak base(ppt actually), strong acid, acidic. Voila! works every time.
And I think it is much simpler than trying to understand which is the stronger base/acid by Bronsted-Lowry theory.