Acid Strength & pH

[plzNOCarribbean]

Just a couple of ? regarding Acid-Base chemistry. I want to make sure I get this right...did a search and found nothing on the topic.

So, when a question asks which of the following is the strongest acid, and lists the concentration of the acid, you would NOT worry about the concentration listed in the answer choices right? Acid Strength is only a measure of an acid's ability to dissociate in water, (ie its a measure of it Ka value)? can one definitively assume this rule for the MCAT?

However, the pH of a solution will depend on BOTH the Concentration of the acid AND the acids ability to dissociate (Ka), right? (which is why when finding the [H+] of a week acid we take the sqrt of Ka x [HA]initial.

Lastly, is it also true that the volume of a solution will NOT affect the pH, unless you are mixing one solution with another. I read something along the line of this in TBR, but I just want to make sure. for example, would this mean that a 100 ml 0.03 M HCL solution has the same pH has a 500 ml 0.03 M HCL solution?


Thanks!

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[vayntraubinator]

1) Acid strength refers to the acids pKa unless given specifics about the volume and the number of moles, I would assume that for the MCAT.


2) Yes, those two scenarios have the exact same pH because it's the same acid and same Molarity.

However, if there were the same number of moles in 100mL and 500mL(say we add 400mL), the 500mL solution would be more diluted and hence, have a pH >**.03M.

Molarity: moles/Liter

Just a couple of ? regarding Acid-Base chemistry. I want to make sure I get this right...did a search and found nothing on the topic.

So, when a question asks which of the following is the strongest acid, and lists the concentration of the acid, you would NOT worry about the concentration listed in the answer choices right? Acid Strength is only a measure of an acid's ability to dissociate in water, (ie its a measure of it Ka value)? can one definitively assume this rule for the MCAT?

However, the pH of a solution will depend on BOTH the Concentration of the acid AND the acids ability to dissociate (Ka), right? (which is why when finding the [H+] of a week acid we take the sqrt of Ka x [HA]initial.

Lastly, is it also true that the volume of a solution will NOT affect the pH, unless you are mixing one solution with another. I read something along the line of this in TBR, but I just want to make sure. for example, would this mean that a 100 ml 0.03 M HCL solution has the same pH has a 500 ml 0.03 M HCL solution?


Thanks!

 

[MShopes]

Just a couple of ? regarding Acid-Base chemistry. I want to make sure I get this right...did a search and found nothing on the topic.

So, when a question asks which of the following is the strongest acid, and lists the concentration of the acid, you would NOT worry about the concentration listed in the answer choices right? Acid Strength is only a measure of an acid's ability to dissociate in water, (ie its a measure of it Ka value)? can one definitively assume this rule for the MCAT?

However, the pH of a solution will depend on BOTH the Concentration of the acid AND the acids ability to dissociate (Ka), right? (which is why when finding the [H+] of a week acid we take the sqrt of Ka x [HA]initial.

Lastly, is it also true that the volume of a solution will NOT affect the pH, unless you are mixing one solution with another. I read something along the line of this in TBR, but I just want to make sure. for example, would this mean that a 100 ml 0.03 M HCL solution has the same pH has a 500 ml 0.03 M HCL solution?
Thanks!

No, acid strength does not depend on Ka alone. It also depends on its pH and therefore concentration of the H+ ions. The more of H+ ions produced, the greater is the dissociation of the acid and therefore the greater the Ka and therefore the lower the ph as well. You have to understand that pH and concentration here does not talk about acid concentration but instead the H+ concentration or H3O+ ions concentration produced from the dissociation of this acid. So concentration and pH matters as well for strength of the acid. For example, HCl is a strong acid and therefore its pH is from 0-1.

Second, the ph depends on the concentration of H+ ions produced like I stressed above not concentration of the acid. And also ph depends on the Ka, the greater the Ka, the greater the H+ produced and the lower the pH.

And yes volume of ONE SOLUTION will not affect the ph because from the equation Moles=Volume * concentration, we can see that the more volume added, the greater the moles as well, so concentration will stay constant regardless of what volume is there. However, volume will affect the pH if one solution is mixed with another because then we will have two different solutions with different moles and therefore final concentration will be different and hence the pH will be affected.

Hope this helps!

 

[jmj33]

Just echoing what other people have said, specifically the part about whether a "pH of a solution is independent of volume."

This is true IF AND ONLY IF the concentration stays the same. Of course 100ml of a .2M HCl solution will have the same pH as 500mL of a .2M HCl, but that is because the moles of acid increase proportionally to the volume. Watch out for a question like "What will be the pH of solution of 200ml .5M HCl after 300ml of water is added?" You may be tempted to jump right to the conclusion that "pH is independent of volume," but in this case of course it is not. It is a simple dilution problem, but it could be tricky if you get stuck thinking that volume is irrelevant to pH

 

[plzNOCarribbean]

No, acid strength does not depend on Ka alone. It also depends on its pH and therefore concentration of the H+ ions. The more of H+ ions produced, the greater is the dissociation of the acid and therefore the greater the Ka and therefore the lower the ph as well. You have to understand that pH and concentration here does not talk about acid concentration but instead the H+ concentration or H3O+ ions concentration produced from the dissociation of this acid. So concentration and pH matters as well for strength of the acid. For example, HCl is a strong acid and therefore its pH is from 0-1.

Second, the ph depends on the concentration of H+ ions produced like I stressed above not concentration of the acid. And also ph depends on the Ka, the greater the Ka, the greater the H+ produced and the lower the pH.

And yes volume of ONE SOLUTION will not affect the ph because from the equation Moles=Volume * concentration, we can see that the more volume added, the greater the moles as well, so concentration will stay constant regardless of what volume is there. However, volume will affect the pH if one solution is mixed with another because then we will have two different solutions with different moles and therefore final concentration will be different and hence the pH will be affected.

Hope this helps!

I appreciate the help, but a couple of things, and I am not trying to be a jerk but maybe someone else could chime in and help.

#1)It also depends on its pH and therefore concentration of the H+ ions.
I don't think acid strength depends on pH, but the other way around. The strength of an acid is solely a measure of an acids ability to dissociate/ionize in solution. The pH is more like the RESULT of the acid dissociating, and the pH is associated with the acidity of the SOLUTION.

So I think it would be correct to say that the strength of acid depends on its Ka (the degree to which that acid dissociates) and the pH of the solution depends on the:
1. The Strength of the acid (an unique, intrinsic property of each acid)
2. The concentration of the acid present

Also.... and again maybe I am wrong about this but

you said moles=Volume * concentration, we can see that the more volume added, the greater the moles as well, so concentration will stay constant regardless of what volume is there

okay, I see that when considering ONE aqueous solution, with no other components besides the acid and water, that the concentration of that acid will remain constant, when nothing else is added. However, from your equation above, and the fact that you said mole = V x concentration, I don't think it holds true that the more "volume is added, the greater the moles as well". If more volume is added, the concentration will be decreased/diluted. This makes sense when you re-arrange your equation to

mole/volume = Concentration; thus Increasing volume DECREASES THE CONCENTRATION.

correct me if I am wrong but I think those statements are right.

 

[plzNOCarribbean]

Just echoing what other people have said, specifically the part about whether a "pH of a solution is independent of volume."

This is true IF AND ONLY IF the concentration stays the same. Of course 100ml of a .2M HCl solution will have the same pH as 500mL of a .2M HCl, but that is because the moles of acid increase proportionally to the volume. Watch out for a question like "What will be the pH of solution of 200ml .5M HCl after 300ml of water is added?" You may be tempted to jump right to the conclusion that "pH is independent of volume," but in this case of course it is not. It is a simple dilution problem, but it could be tricky if you get stuck thinking that volume is irrelevant to pH

Right, because a tank holding 100 ml of water with a .2M HCL will have the same [H] concentration as a 500 ml tank with a .2 M HCL , because in both instances the acid will dissociate to the same extent and the the [H+] in each solution is the same, so the pH of each solution is the same. Is this correct?

#2) Great example BTW-"What will be the pH of solution of 200ml .5M HCl after 300ml of water is added?" is there 2 ways to solve this?

could you do, M1V1 = M2V2 and set it up like this:
(200ml)(.5M)= (M2)(500)
M2= .2M = [H+]

Could you multiple (.5M)(0.2L) to find the total # of moles in solution (.1 moles) and divide this by the TOTAL VOLUME, .5L?

Would both of these methods be appropriate in solving the question above? Thanks again for your help.

 

[gunj122]

acid strength is a function of the Ka. it is defined as the ability of the molecule to dissociate.

the pH is affected by the Ka but the Ka is NOT affected by the pH. the only thing that affects Ka is temperature, as you should know from dealing w/ Keq, which is the same thing but a broader term.