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Determine whether compound will be acidic, basic, or neutral when dissolved in water

Submitted by WilliamC on Sat, 04/05/2008 - 21:20

This question has me stumped.

Predict whether aqueous solutions of the following substances are acidic, basic, or neutral:

LiI
K3PO4
[CH3NH3]Cl

I found the portions of my text that deal with this problem, but it doesn't seem to go into enough detail. I know for these problems, you can sometimes ignore parts of the solution for some reason. I understand that the first step is to break each solution down into cations and anions, then they need to be analyzed individually.

So, breaking K3PO4 apart, I should get K (with a +3 superscript), which means PO4 should have a -3 superscript (I think). Now at this point I am supposed to know if these cations and anions are part of strong or weak acids or bases to determine the answer? Is there a chart for this somewhere?


Here are some general rules:
1) Salt of a strong Acid + Strong base---->Neutral Solution
2) Salt of a Strong Base+ Weak acid-----> basic solution
3) Salt of a Strong Acid + Weak base----> acidic solution

YOU MUST MEMORIZE STRONG ACIDS AND STRONG BASES: everything other than these are considered as weak

Strong acids are: HCl, HBr, HI, HClO3, HClO4, HNO3, HIO4, H2SO4
Strong bases are: All Hydroxides of Group I (NaOH, KOH, etc) and hydroxides of Mg, Ca, Sr, Ba.

Here are a few examples

HCl + NaOH----> NaCl + H2O
(SA)  (SB)          neutral

CH3COOH + NaOH----> NaCH3COO  +H2O
  (WA)          (SB)          basic

HCl + NH3----> NH4Cl
(SA)  (WB)      acidic

I'll help you on the first one: LiI
since Li+ is the cation, it must have been basic LiOH (it couldn't be acidic because H forms a positive cation and Li + is a positive cation so they can't go together)

And since I- is the anion, it must have been from HI. why? simple: H+ + I-  = HI

So now we can write:  LiOH + HI-----> LiI + H20

so we have a strong base + a weak acid, which yields a basic solution. SO, LiI is basic

and as for your reasoning as K+3, this is incorrect. K ALWAYS has a +1 charge. The phosphate ion and K ion merely "swapped" charges. because PO4 has a charge of -3, it takes 3 K ions to neutralize the charge, thus, there are 3 K ions per phosphate ion: K3PO4


"and as for your reasoning as K+3, this is incorrect. K ALWAYS has a +1 charge. The phosphate ion and K ion merely "swapped" charges. because PO4 has a charge of -3, it takes 3 K ions to neutralize the charge, thus, there are 3 K ions per phosphate ion: K3PO4"

I think that was what I was attempting to get across. I know that K has a plus 1 charge because its in the first group- but since there was 3 of them, I felt the total charge was plus 3. And then I figured for the substance to be neutral, PO4 had to have a plus 3 charge. This isnt correct?

By the way thanks for the help, I will definitely memorize the strong acids and bases!


I think the problem was not in your logic which is correct (there are 3 K+ ions, each with a charge of +1, therefore the PO4-3 must have a -3 charge).  By stating that K has a superscript of +3, Kyle thought that you meant that the charge on each potassium ion was +3.  To avoid this problem in the future, remember that charges written as superscripts always apply to single atoms.


I'm a bit confused on the LiI example after doing all these. I should start by writing it as:

LiI + H2O-->LiOH + HI

Now, LiOH is a strong base, but HI is also a strong acid...

so should the answer be neutral instead of basic?


LI is a soluble salt

so it will dissociate in water to Li+  and I-

I- is the conjugate base to the acid HI

so it will be basic

I- + H2O ---> HI + OH-


I'm still little confused on LiI.
When you think about Li+,
Isn't  Li+  the conjugate acid of LiOH?


think of conjugate acids and bases when considering weak acids and bases.

LiOH is a soluble salt, there is no reaction in water, it is simply a dissociation.