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

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?

Re: Determine acidic, basic, or neutral

kyle1990 Sat, 04/05/2008 - 23:23

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

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

Re: Determine acidic, basic, or neutral

WilliamC Sun, 04/06/2008 - 09:06

"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!

Re: Determine acidic, basic, or neutral

spock Sun, 04/06/2008 - 12:41

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.

Re: Determine acidic, basic, or neutral

WilliamC Mon, 04/07/2008 - 19:14

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...

Re: Determine acidic, basic, or neutral

Chemistry Tutor Mon, 04/07/2008 - 20:03

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-

Re: Determine acidic, basic, or neutral

YUISA Wed, 04/09/2008 - 12:52

I'm still little confused on LiI.