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:
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.