Sapling #13

[haley f]

A monoprotic weak acid, HA , is ionized according to the reaction

HA(aq)+H2O(l)↽−−⇀A−(aq)+H3O+(aq)pKa=4.08

where A− is the conjugate base to HA.
For this weak monoprotic acid, the predominant species present at pH 2.32 is: unknonw, charged, or neutral

I know that when pH is less than pKa HA will become more prominent, but does that mean it is neutral or charged?

[Rylee Mangan 1K]

When pH > pKa then the acid will be negatively charged.
When pH < pKa then the acid will be neutral.

[haley f]

When pH > pKa then the acid will be negatively charged.
When pH < pKa then the acid will be neutral.

That makes sense, thank you!

[905565862]

What is the distinguishing factor between pKa and pH?

[Brooklyn Burgess 3L]

What is the distinguishing factor between pKa and pH?

pKa is the acid dissociation constant (in aqueous solution) and is defined by -log[Ka] and indicates whether an acid is strong or weak. pH indicates whether a system is acidic or alkaline and is defined by -log[H+].

The smaller the pKa value, the stronger the acid. If the pH of a system is high, the system is alkaline, but if the pH is low, that system is acidic.

[Sydney Rohan 3G]

When pH > pKa then the acid will be negatively charged.
When pH < pKa then the acid will be neutral.

I am confused. I use this same thought process on this problem:

A weak base, B, is ionized forming its conjugate acid ( BH+ ) according to the reaction
B(aq)+H2O(l) <----> BH+(aq)+OH−(aq)
The conjugate acid BH+ has a pka=8.98.
For this weak base, the predominant species present at pH 6.04 is
- neutral
-charged
-unknown

The pKa would be 8.98 and the pH would be 6.04, so the pKa>pH. Therefore, if you use your reasoning, then the answer would be that the "neutral" but the correct answer is "charged," so I am confused about how to find the right answer. Can you please help me?

[Jessie_Ehret_1B]

When pH > pKa then the acid will be negatively charged.
When pH < pKa then the acid will be neutral.

I am confused. I use this same thought process on this problem:

A weak base, B, is ionized forming its conjugate acid ( BH+ ) according to the reaction
B(aq)+H2O(l) <----> BH+(aq)+OH−(aq)
The conjugate acid BH+ has a pka=8.98.
For this weak base, the predominant species present at pH 6.04 is
- neutral
-charged
-unknown

The pKa would be 8.98 and the pH would be 6.04, so the pKa>pH. Therefore, if you use your reasoning, then the answer would be that the "neutral" but the correct answer is "charged," so I am confused about how to find the right answer. Can you please help me?

Hi!

So the reason that for this question, it is the opposite is because you're dealing with a weak base rather than a weak acid. For bases, when pH > pKa the predominant species is the neutral, deprotonated species (B) while when pH < pKa the predominant species is the charged, protonated species BH+. Hope that helps clarify!

[Caleb Kim 1K]

It is neutral. Think of it as two separate solutions in a tank. One being the acid and the other being the solution. When the acid has a pka greater than the pH of the solution, the acid isn't more acidic than the solution, meaning it will fail to give up its proton to the solution. Because of this, HA will remain neutral. Hope this helps!