Making solution of desired pH

[anbuitachi]

---

Members don't see this ad.

I have 1 mole of H2CO3. I want to make the solution pH 6. Do I add strong acid or strong base?

Answer: Strong base

Question:

I understand why the answer is strong base, however my question is a weak base one that has a high pH? (ie people say low pH is strong acid and vice versa), or is a weak acid simply one that doesn't dissociate fully? Because having only conjugate acid pushes pH much lower than pKa, this making pH super low... so im a bit confused

 

[Kimchii]

I have 1 mole of H2CO3. I want to make the solution pH 6. Do I add strong acid or strong base?

Answer: Strong base

Question:

I understand why the answer is strong base, however my question is a weak base one that has a high pH? (ie people say low pH is strong acid and vice versa), or is a weak acid simply one that doesn't dissociate fully? Because having only conjugate acid pushes pH much lower than pKa, this making pH super low... so im a bit confused

Your question is a bit confusing, but I'll try to answer it. Strong refers to fully dissociate while weak refers to partially dissociate depending on the pKa. A weak base isnt one that has high pH. It can't "have" a high pH it "creates" the pH and it wont make it a high pH unless there is a high concentration of it because there is less binding of H ions to it. You can reach a high pH solution two ways: 1. use a strong base regardless of concentration, 2. use a weak base in a very high concentration because a weak base only partially depletes the concentration of H ions in solution.

 

[anbuitachi]

Basically im saying based on the H-H equation.

ph= pka + log B/A

if you dump a weak acid, it will dissociate but since it is weak, most of it will remain as acid with very little base, thus A >>>B

So lets say the pKa of the weak acid is 5.

pH = 5 + log (small #/Large #)

Assume it dissolves 1 to 1000

pH = 5 + log (1/1000) = 2

So basically im a bit confused b/c it is a weak acid yet the pH is 2 ...It just doesn't sound right to me but based on the equation it should be right

 

[indianjatt]

Basically im saying based on the H-H equation.

ph= pka + log B/A

if you dump a weak acid, it will dissociate but since it is weak, most of it will remain as acid with very little base, thus A >>>B

So lets say the pKa of the weak acid is 5.

pH = 5 + log (small #/Large #)

Assume it dissolves 1 to 1000

pH = 5 + log (1/1000) = 2

So basically i'm a bit confused b/c it is a weak acid yet the pH is 2 ...It just doesn't sound right to me but based on the equation it should be right

I think you are confused between strong acid/base and pH. A strong acid or a base (bronsted) will dissociate readily in an aqueous solution and produce a lot of H+ charges in solution for an acid and OH- for a base. As a result, they will have LOWER pHs and pOHs respectively because there are more H+ and OH- ions in solution. pH doesn't necessarily tell you if an acid is strong or not, but rather the concentration of the hydronium ion and hydroxide ion in solution. So yes, a weak acid can have a pH if you have ENOUGH weak acid in solution to create a 10^-2M solution of hydronium ions.

Sorry if I was unclear

 

[BerkReviewTeach]

Basically im saying based on the H-H equation.

ph= pka + log B/A

if you dump a weak acid, it will dissociate but since it is weak, most of it will remain as acid with very little base, thus A >>>B

So lets say the pKa of the weak acid is 5.

pH = 5 + log (small #/Large #)

Assume it dissolves 1 to 1000

pH = 5 + log (1/1000) = 2

So basically im a bit confused b/c it is a weak acid yet the pH is 2 ...It just doesn't sound right to me but based on the equation it should be right

The problem is that you are using the HH equation on a system where it is not applicable. In order for the HH equation to be valid [HA] must be close to [A-], which isn't the case as you've presented it.

In school they taught you to do the ICE box method and solve for x. The is is the [H+] used in the pH = -log [H+] relationship. If you are using BR books, then you should use the short cut equation (which is basically the ICE box result with a couple twists). The shortcut equation only includes [HA] and pKa because you've only added the weak acid.