How can a weak base completely dissociate?

[fas376]

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This is from TBR Gen Chem. The answer states "Both KOH and KOAc are salts. As such, they will both completely dissociate into water, so the ion concentration is the same in both solutions."

Can someone shed some light on this? I thought only strong bases and acids completely dissociated, and the text states that KOAc is a weak base.

 

[MedPR]

This is from TBR Gen Chem. The answer states "Both KOH and KOAc are salts. As such, they will both completely dissociate into water, so the ion concentration is the same in both solutions."

Can someone shed some light on this? I thought only strong bases and acids completely dissociated, and the text states that KOAc is a weak base.

If I recall that passage correctly, there is a figure that tells you the electric conductivity of KOH and KOAc are just about equal. Based on that alone you can tell that KOAc must be dissociating completely because you know KOH is a strong base and will also dissociate completely.

Just because something is a weak base doesn't mean it won't dissociate completely. Ionic salts will dissociate completely in water because water is polar. Like dissolves like.

 

[fas376]

If I recall that passage correctly, there is a figure that tells you the electric conductivity of KOH and KOAc are just about equal. Based on that alone you can tell that KOAc must be dissociating completely because you know KOH is a strong base and will also dissociate completely.

Just because something is a weak base doesn't mean it won't dissociate completely. Ionic salts will dissociate completely in water because water is polar. Like dissolves like.

Isn't the fundamental difference between a strong and weak base the amount of dissociation it undergoes? So if a weak base completely dissociates, shouldn't it be considered strong? Or am I missing some fundamental concept here...

 

[MedPR]

Isn't the fundamental difference between a strong and weak base the amount of dissociation it undergoes? So if a weak base completely dissociates, shouldn't it be considered strong? Or am I missing some fundamental concept here...

NH3 is a weak base. KOAc is a salt that is weakly basic.

Typically people make the distinction that strong acid/base = dissociate completely, and weak acid/base do not. This distinction has clearly led you in the wrong direction, so I tried to reword it to help. In other words, a weakly basic salt is not the same as a weak base.

What did you learn about ionic compounds before you ever learned about acids and bases? You learned that they are polar, and will dissolve very well in polar solvent. Since KOAc is an ionic compound, you would expect it to dissociate pretty well in water.

 

[milski]

Salts can dissociate completely.

That's different from how much the acid/base itself will dissociate.

 

[fas376]

NH3 is a weak base. KOAc is a salt that is weakly basic.

Typically people make the distinction that strong acid/base = dissociate completely, and weak acid/base do not. This distinction has clearly led you in the wrong direction, so I tried to reword it to help. In other words, a weakly basic salt is not the same as a weak base.

What did you learn about ionic compounds before you ever learned about acids and bases? You learned that they are polar, and will dissolve very well in polar solvent. Since KOAc is an ionic compound, you would expect it to dissociate pretty well in water.

Aw, man. I think I figured out the source of my problem. I'm trying to "connect" everything and assumed that salts don't dissociate if they have low Ksps. For example, MgCO3 has a very low Ksp (1.1E -15) so I assumed it won't dissociate in solution. That's not right, is it? Damn

 

[MedPR]

Aw, man. I think I figured out the source of my problem. I'm trying to "connect" everything and assumed that salts don't dissociate if they have low Ksps. For example, MgCO3 has a very low Ksp (1.1E -15) so I assumed it won't dissociate in solution. That's not right, is it? Damn

MgCO3 is insoluble in water as indicated by the low Ksp value. I'd imagine the Ksp for potassium acetate is significantly larger than the Ksp for MgCO3.

 

[fas376]

MgCO3 is insoluble in water as indicated by the low Ksp value.

But it's an ionic salt, so it should completely dissociate like you said, right?

 

[MedPR]

But it's an ionic salt, so it should completely dissociate like you said, right?

Yes it should, but if the Ksp given is accurate, it is insoluble and is probably one of the exceptions.

 

[MedPR]

Actually, almost all carbonates are insoluble. I think K2CO3, Na2CO3 and (NH4)2CO3 are the only soluble carbonates.

But NH4+, Na+, and K+ are almost always soluble regardless of their anion. Just like how NO3- and SO42- are typically soluble with a few exceptions.

There are enough exceptions in solubility to make it a topic in itself.

 

[fas376]

Actually, almost all carbonates are insoluble. I think K2CO3, Na2CO3 and (NH4)2CO3 are the only soluble carbonates.

But NH4+, Na+, and K+ are almost always soluble regardless of their anion. Just like how NO3- and SO42- are typically soluble with a few exceptions.

There are enough exceptions in solubility to make it a topic in itself.

Sounds good. Thanks for your help mate.

 

[MedPR]

Sounds good. Thanks for your help mate.

Happy to help. Thanks for reminding me that I need to look at solubility rules. I don't know when your test date is, but by the time 3 months comes around, you might find yourself overlooking the more basic topics (like solubility rules). Remember to review them!

 

[Class1P]

I think it is kind of a mistake to say a Base dissociates completely.

The base would be the -OAc part of the Salt. The Base doesn't dissociate per se.

I'd be more inclined to think of it in terms of proton snatching from water.

A Strong Acid HA + H20 --> H30+ + A-
produces the entirety of it's original moles into protons given up.

A strong base -OH + H20 ---> OH- + HA
absorbs protons equal to it's initial concentration.

In the case that you have a Salt of a weak base KOAc + H20 --> HOAc + KOH
You only get a partial quantity of the salt absorbing a proton.

If you assume that it is completely soluble, your KOAc would dissociate entirely into K+ and OAc- Ions. BUT only a fraction of those OAc- Anions will act as a base.

 

[MrNeuro]

All acetates are soluble... I didn't read what everyone else wrote but that's what I thought of.