Why doesn't the # of ions in solution DOESN'T depend on the acid/base strength?

[sc2016]

The strength of an acid or base (whether it is weak or strong) affects its dissociation in solution and so the # of ions in the solution. So why doesn't the number of ions depend of the acid/base strength?

(BR Chem Ch.4 passage 2 Q9)

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[Rabolisk]

It would.. I think you have to post the question and the relevant information from the passage for me to answer.

 

[Donald Juan]

The question is comparing two salts with a similar molar solubility. One of the salts is KOH, a strong base, while the other is KCH3COO, a weak base. However, they both fully dissociate and therefore have a similar number of ions in the water. The fact that KOH is a stronger base does not affect this.

 

[sc2016]

It would.. I think you have to post the question and the relevant information from the passage for me to answer.

Thats what I thought.

The questions is how can it be explained that there is no difference
between the current readings in Trial 5 and Trial 6?

In each trial 25ml of aliquots of acid are added and the current is measured current is measured.
Trial 5 — 0.10 M KOH — gives a current 5.89 amps
Trial 6 — 0.10 M KOAc — gives a current 5.92 amps



And the answer is that the number of ions in solution does not depend on
the base strength.

 

[Donald Juan]

was my explanation not clear?

Consider NaCl, a neutral salt. If we put it into water it full dissociates and we have a large number of ions in solution, even though you have an extremely weak acid/base pair. The number of ions in the solution do not depend on the strength of the base.

 

[Rabolisk]

Since the current is more or less equal, the number of ions in solution must be equal. Simple as that.

To go a little bit further, both salts dissociate fully. Although CH3COO- is a much weaker base than OH-, it doesn't matter, because in both cases, the acid/base reaction doesn't change the number of ions.

CH3COO- + H2O --> CH3COOH + OH-

Regardless of what the Kb of CH3COO- is, the number of ions cannot change. That is, it doesn't matter where the equilibrium lies.

 

[sc2016]

was my explanation not clear?

Consider NaCl, a neutral salt. If we put it into water it full dissociates and we have a large number of ions in solution, even though you have an extremely weak acid/base pair. The number of ions in the solution do not depend on the strength of the base.

Since the current is more or less equal, the number of ions in solution must be equal. Simple as that.

To go a little bit further, both salts dissociate fully. Although CH3COO- is a much weaker base than OH-, it doesn't matter, because in both cases, the acid/base reaction doesn't change the number of ions.

CH3COO- + H2O --> CH3COOH + OH-

Regardless of what the Kb of CH3COO- is, the number of ions cannot change. That is, it doesn't matter where the equilibrium lies.

Thank you Donald Juan and Rabolisk, it makes so much sense now. Sorry it took me a while to understand.

 

[Integrated MCAT Course]

I think this question is flawed. My understanding is that salts have different molar conductivities not only due to Van't Hoff factor, or degree of dissociation, but also due to intrinsic differences in the free migration of the ions themselves in solution which is reflected in the Kohlrausch coefficient. Am I wrong about this? I think using a colligative property such as boiling point elevation would have been a better way to get at the understanding that both are strong electrolytes than conductivity reflected in amperage. Different ions have different Scopes radius and different hydration shell dynamics. I just don't think that even at nearly infinite dilution the molar conductivity of different ions is the same. Because this is from TBR I find myself doubting myself, but just a little bit.

 

[loveoforganic]

John, you're right, but that's beyond the scope of the MCAT as far as I'm aware

 

[Donald Juan]

I think this question is flawed. My understanding is that salts have different molar conductivities not only due to Van't Hoff factor, or degree of dissociation, but also due to intrinsic differences in the free migration of the ions themselves in solution which is reflected in the Kohlrausch coefficient. Am I wrong about this? I think using a colligative property such as boiling point elevation would have been a better way to get at the understanding that both are strong electrolytes than conductivity reflected in amperage. Different ions have different Scopes radius and different hydration shell dynamics. I just don't think that even at nearly infinite dilution the molar conductivity of different ions is the same. Because this is from TBR I find myself doubting myself, but just a little bit.

Like the previous poster said, beyond the scope.

Also, the question is just asking for an explanation as to why the two different bases could have similar conductivity, and the correct answer is the only one that offers a fair explanation (I looked it up in my book this afternoon). It comes down to one of those "choose the best answer" types of things.