Berkley Review Buffer Contradiction??

[johnwandering]

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I am having trouble understanding a concept.

When Water is added to a buffer solution of pH=3.96, the pH doesn't change
But when 5 drops of 12M HCl is added to a solution of pH 8.31, the pH drops to 8.26...

Why are these different exactly?
I understand that HCl is a wildly powerful acid, But the issue is


If I am ever confronted with a question of this type involving an acid of "Medium strength" between these, how will I know if the pH will change??
(No real numbers were given me so that I can calculate HH equation)

 

[PiBond]

I am having trouble understanding a concept.

When Water is added to a buffer solution of pH=3.96, the pH doesn't change
But when 5 drops of 12M HCl is added to a solution of pH 8.31, the pH drops to 8.26...

Why are these different exactly?
I understand that HCl is a wildly powerful acid, But the issue is


If I am ever confronted with a question of this type involving an acid of "Medium strength" between these, how will I know if the pH will change??
(No real numbers were given me so that I can calculate HH equation)

H20 won't change pH because we haven't affected the ratio of A-/HA. The HH equation only depends on pka (a constant) and the ratio of conjugate base to conjugate acid. Thus, both haven't changed in the equation and therefore pH doesn't either

12M HCl is very concentrated so it'll absolutely drop pH because it's increasing [H+]. Because it's only a few drops BR is trying to get you to ballpark the correct answer. I bet you can probably eliminate 2 answers just by the sheer fact that the pH is decreasing and the other is probably way too large of a drop.

 

[johnwandering]

Thank you!



But what prevents the H20 from deprotonating the acid and hence raising the [A-]/[HA] value??

I know that the is not a significant amount,
But if there is any deprotonation whatsoever, then you have to admit that the pH is rising slightly~~~


=(

 

[PiBond]

Thank you!



But what prevents the H20 from deprotonating the acid and hence raising the [A-]/[HA] value??

I know that the is not a significant amount,
But if there is any deprotonation whatsoever, then you have to admit that the pH is rising slightly~~~


=(

you want your buffer to be in roughly equal concentrations of weak acid and its conjugate base. water couldn't deprotonate a weak acid

 

[lesset01]

Thank you!



But what prevents the H20 from deprotonating the acid and hence raising the [A-]/[HA] value??

I know that the is not a significant amount,
But if there is any deprotonation whatsoever, then you have to admit that the pH is rising slightly~~~


=(

Water does deprotonate the weak acid, but it also protonates the conjugate base by an equivalent amount. So the ratio of acid to conjugate base remains the same.

 

[johnwandering]

So I should remember that:


If any strong acid/base added to buffer ----> pH change slightly
If any weak acid/base added to buffer -----> pH is constant


is this correct??

 

[BerkReviewTeach]

So I should remember that:


If any strong acid/base added to buffer ----> pH change slightly
If any weak acid/base added to buffer -----> pH is constant


is this correct??

Not quite.

As pi bond said, the key thing is to consider the [A-]/[HA] ratio. Anything that changes that ratio will affect the pH.

The HH equation is pH = pKa + log [A-]/[HA]

If strong acid is added, then A- turns into HA, so [A-]/[HA] goes down causing the pH to go down, but slightly.

If strong base is added, then HA turns into A-, so [A-]/[HA] goes up causing the pH to go up, but slightly.

If weak acid is added, then [HA] goes up, causing [A-]/[HA] to go down and thus the pH goes down slightly.

If weak base is added, then [A-] goes up, causing [A-]/[HA] to go up and thus the pH goes up slightly.

It's important that a buffer resists drastic pH changes when an acid or base is added, whether it's weak or strong. But the pH still changes with the addition of an acid or a base, but only slightly.