Julie18
Harmless
Posts: 5
Registered: 16-6-2009
Member Is Offline
Mood: Scrumdiddlyumptious.
|
|
Basic solutions?
How do you know when something forms a basic solution when dissolved in water if it doesn't have an OH?
This is the question I'm looking at:
Which of the following salts forms a basic solution when dissolved in water?
(A) NaCl
(B) (NH4)2SO4
(C) CuSO4
(D) K2CO3
(E) NH4NO3
Thanks ahead of time
|
|
|
kclo4
National Hazard
Posts: 916
Registered: 11-12-2004
Location:
Member Is Offline
Mood: No Mood
|
|
If you plan on asking multiple questions like this and the other, instead of starting multiple threads you could place them all in one, or post them
in the short questions thread that we have around here.
K2CO3 forms a basic solution.
|
|
|
Zarcorp
Harmless
Posts: 15
Registered: 28-4-2009
Member Is Offline
Mood: No Mood
|
|
I imagine it's along the lines of looking for reactions that would produce a hydroxide.
I really don't know much of anything though.
|
|
|
12AX7
Post Harlot
Posts: 4803
Registered: 8-3-2005
Location: oscillating
Member Is Offline
Mood: informative
|
|
Anions with a fairly high pKa react with water to form some concentration of hydroxyls (lowering pOH / raising pH). In general, the pH you'll get is
approximately (give or take a pKa unit) the pKa of the anion. (It is exactly equal if you add equal parts salt and acid, forming a buffer solution.
This assumes the salt is with a strong cation, like Na, but generally not NH4.)
Tim
|
|
|
JohnWW
International Hazard
Posts: 2849
Registered: 27-7-2004
Location: New Zealand
Member Is Offline
Mood: No Mood
|
|
The answer to that question is D, K2CO3, because it is the salt of a strong mineral base and a weak acid, the latter being liable to undergo partial
hydrolysis to H2CO3, which removes H+ ions from solution.
|
|
|
woelen
Super Administrator
Posts: 7977
Registered: 20-8-2005
Location: Netherlands
Member Is Offline
Mood: interested
|
|
In general, salts consist of at least two ionic species, the cation(s) and the anion(s). Look at the cationic species and at the anionic species.
Now think of the salt solution being formed of the cation hydroxides and the acids, based on the anion. Then determine the strength of all the bases
and the strength of the acids. If you have done that, then you can say the following in general:
- salt, based on only strong bases and strong acids --> neutral or near neutral solution
- salt, based on strong bases and weak acids --> alkaline solution
- salt, based on weak bases and strong acids --> acidic solution
- salt, based on weak bases and weak acids --> cannot simply be determined, relative weaknesses of acid and base must be determined.
Some examples:
Ammonium chloride: NH4Cl
Based on NH4OH(better: NH3 + H2O) and HCl. Weak base and strong acid. The solution is acidic.
Sodium acetate: NaCH3COO
Based on NaOH and CH3COOH. Strong base and weak acid. The solution is alkaline.
Magnesium chloride: MgCl2
Based on Mg(OH)2 and HCl. Strong base (although fairly insoluble) and strong acid. The solution is (near) neutral.
Potassium aluminium sulfate: KAl(SO4)2
Based on KOH, Al(OH)3 and H2SO4. Strong base, weak base and strong acid. Strong base and weak base makes total weak base. Solution hence is acidic.
Ammonium acetate: NH4CH3COO
Based on NH3+H2O and CH3COOH. Without precise tables of strength of acids and base one cannot simply say whether this solution is acidic or alkaline.
But, one thing is for sure, the solution is less alkaline than a solution of NaCH3COO and the solution is less acidic than a solution of NH4Cl. So, it
will be closer to neutrality, but the final answer can only be given if the pKz and pKb of the acid and base are known.
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
