TBR Chem Chapter 4 # 46

[brood910]

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If a weak acid is titrated with enough strong base so that [A-] > [HA], then for the resulting compound:
A) [H3O+] < Ka
B) pH<pKa
C) [H3O+] > [A-]
D) [HA] < [H3O]

It was easy to get the answer, which is A. But I want to know WHY C and D are wrong. Their explanations are really confusing.

 

[Jepstein30]

Well, if C is right.. then D is right also since [A-] > [HA] .. so C cannot be right.
For D, we have to consider the only source of H3O here which would be whatever leftover weak acid (very little) reacting with water (HA + H2O <-> H3O + A-). Weak acids don't dissociate very much, so the reactants will be favored. If it was a strong acid, the products would be favored and D would be correct.

Makes sense to me but I'm sure there's another way of looking at it.

 

[brood910]

Thanks for the answer. That's what I figured out for D, but I still do not understand C.

I know that [A-] = [H3O+] for weak acid, but is this also true after being titrated with a strong base??

 

[Jepstein30]

I didn't explain C conceptually but that reasoning works.

Conceptually, similar idea.
Two reactions:
HA + OH- --> H2O + A- (weak acid titration)
HA + H2O <-> H3O + A- (weak acid dissociation)

The first reaction is heavily favored, the second isn't. The only way we get H3O is via the second reaction.. which would produce the same amount of A-. The first reaction, though, goes until A- > HA.. meaning more than half the weak acid is titrated into its conjugate. We get a ton of A- from the first reaction. Since the second reaction produces the exact same amounts of both H3O and A-, this second source of A- makes it so A- must be > than H3O.

 

[4563728]

.

 

[Czarcasm]

I didn't explain C conceptually but that reasoning works.

Conceptually, similar idea.
Two reactions:
HA + OH- --> H2O + A- (weak acid titration)
HA + H2O <-> H3O + A- (weak acid dissociation)

The first reaction is heavily favored, the second isn't. The only way we get H3O is via the second reaction.. which would produce the same amount of A-. The first reaction, though, goes until A- > HA.. meaning more than half the weak acid is titrated into its conjugate. We get a ton of A- from the first reaction. Since the second reaction produces the exact same amounts of both H3O and A-, this second source of A- makes it so A- must be > than H3O.

You should seriously consider teaching. Your explanations are always so clear and perfect. 🙂

 

[Jepstein30]

You should seriously consider teaching. Your explanations are always so clear and perfect. 🙂

thanks.. well ahead of ya though 😉

 

[drechie]

I didn't explain C conceptually but that reasoning works.

Conceptually, similar idea.
Two reactions:
HA + OH- --> H2O + A- (weak acid titration)
HA + H2O <-> H3O + A- (weak acid dissociation)

The first reaction is heavily favored, the second isn't. The only way we get H3O is via the second reaction.. which would produce the same amount of A-. The first reaction, though, goes until A- > HA.. meaning more than half the weak acid is titrated into its conjugate. We get a ton of A- from the first reaction. Since the second reaction produces the exact same amounts of both H3O and A-, this second source of A- makes it so A- must be > than H3O.

why is the weak acid titration heavily favored and more favored than the weak acid dissociation?

 

[Teleologist]

why is the weak acid titration heavily favored and more favored than the weak acid dissociation?

Well, why not? Whatcha think?

NB: weak acid titrations are not always "heavily favored."

NB2: Strong acids and bases react to an extent of ~100%.

 

[drechie]

Well, why not? Whatcha think?

NB: weak acid titrations are not always "heavily favored."

NB2: Strong acids and bases react to an extent of ~100%.

Oh, so despite "HA" being a weak acid, because it reacts with a strong Base (Hydroxide), it will proceed in the forwards direction?