Picking Strength of base on the real DAT

[gangazi]

Say they give us NaOH, NH3, and HSO4- and tell us to pick the weakest base.

kaplan says eliminate HSO4- since it is an acid
qvault says HSO4- is the weakest base because it's conjugate base of the strong acid

what should i do on the real DAT?

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[JLT223]

Listen to Qvault. I'm not a fan of Kaplan.

 

[Ashish]

For basicity, the approach I take is to determine which specie has readily available electrons that they can "use" to attract Hydrogen. OH- and Nitrogen both have lone pairs that they can use while HSO4- does not ( You can verify this by lewis structure). Thus HSO4- must be the weakest base.

Analogus to this, for acidity, I simply convert each into conjugate base and use the CARDIO rule.

 

[klownzo]

I always like to look at acidity for these questions. Use the CARDIO rule and you'll be set. The stronger the acid, the weaker the base!

 

[aceofbass]

I always like to look at acidity for these questions. Use the CARDIO rule and you'll be set. The stronger the acid, the weaker the base!

Care to explain the CARDIO rule? Thanks

 

[super frank]

Care to explain the CARDIO rule? Thanks

Base strength depends on these factors, in descending order of importance: charge, atom, resonance, dipole, induction, orbitals (CARDIO).

I.e, negative trumps neutral, nitrogen trumps oxygen (periodic trend), more resonance = more stable and thus less reactive (removing electron density from the base), same goes for dipole and induction (removing e- density makes the base less reactive, making it a weaker base, or a stronger acid). Orbitals are a little bit more abstract: the more sp character the electrons have, the less reactive the base. So sp bases are less reactive than sp2, which are less reactive than sp3. For this, think of o-chem, the acetylide ion forms because the terminal H is actually acidic (because its conjugate base is stabilized due to its 50/50 sp character).

 

[DentalNucleicAcid]

Think about these problems from an o/chem perspective. The weakest base (which is also the strongest acid) will be the one that is most stable. Stability comes from resonance. HSO4- has a lot resonance if you draw out the lewis structure. If you're deciding between two things that have no resonance, then you look at charge and electronegativity in that order. The less electronegative will be the the stronger base. But charge beats electronegativity. Don't ask yourself what's a stronger or weaker acid/base; ask yourself, which one of these is more stable? Once you've figured that out, make the connection that the more stable is a weaker base (and stronger acid).

 

[gangazi]

Hmm what about when the pKb is given? what value is a weak/strong base?
on DAT BC FL1, a question asked if the molecule that has conjugate base pKb of 1.4 is strong or weak acid
How do you determine by just using the number? If multiple pKb is given I can compare which conjugate acid is the strongest acid, but given only 1.4 I was hesitant to think it was a strong base

 

[Ashish]

Hmm what about when the pKb is given? what value is a weak/strong base?
on DAT BC FL1, a question asked if the molecule that has conjugate base pKb of 1.4 is strong or weak acid
How do you determine by just using the number? If multiple pKb is given I can compare which conjugate acid is the strongest acid, but given only 1.4 I was hesitant to think it was a strong base

Low pKb means a strong base (almost all of the base dissociates because of high Kb value). So, its conjugate must be weak. So, its weak acid. All p values follow the same trend i.e low pH means acidic, low pOH means basic, low pKa means acidic, low pKb means basic.

 

[gangazi]

Low pKb means a strong base (almost all of the base dissociates because of high Kb value). So, its conjugate must be weak. So, its weak acid. All p values follow the same trend i.e low pH means acidic, low pOH means basic, low pKa means acidic, low pKb means basic.

But whats a low value? Because i thought HCl has like pKa of negative value so I thought 1.4 might not be low enough

 

[Ashish]

But whats a low value? Because i thought HCl has like pKa of negative value so I thought 1.4 might not be low enough

I don't know if there is a numerical cut off value for acidity and basicity in terms of pK values. What we do know is at pH=pka (half equivalence point), half of the specie is dissociated. My guess is that any value approaching zero is surely strong (in terms of pk values).

 

[Queen Beach]

Both Kaplan and Qvault are correct.

For Kaplan, yes you eliminate HSO4- plain and simple bc it's not the weakest base out of those 3. NaOH is a strong base and NH3 is a weak base.

For qvault, hso4 is proton donor

For the real dat i recommed Chad's videos for content review.

Say they give us NaOH, NH3, and HSO4- and tell us to pick the weakest base.

kaplan says eliminate HSO4- since it is an acid
qvault says HSO4- is the weakest base because it's conjugate base of the strong acid

what should i do on the real DAT?

 

[gangazi]

Both Kaplan and Qvault are correct.

For Kaplan, yes you eliminate HSO4- plain and simple bc it's not the weakest base out of those 3. NaOH is a strong base and NH3 is a weak base.

For qvault, hso4 is an acidic anion therefore it will give you a ph greater than 7.

For the real dat i recommed Chad's videos for content review.

 

[gangazi]

@orgoman22 hello Dr. Romano, can you tell us if we should eliminate HSO4- right away if they ask about "pick a weakest base?"

We know you are busy so we will patiently wait!

 

[Queen Beach]

If you would have listened to Dr. R about reading your textbook than you would not be so confused.
For qvault if you think of hso4- as the conjugate base of a strong acid then of the 3, hso4- is the weakest base even though it's acidic if placed in h2o.

 

[super frank]

Hmm what about when the pKb is given? what value is a weak/strong base?
on DAT BC FL1, a question asked if the molecule that has conjugate base pKb of 1.4 is strong or weak acid
How do you determine by just using the number? If multiple pKb is given I can compare which conjugate acid is the strongest acid, but given only 1.4 I was hesitant to think it was a strong base

This is a very basic question. You can either memorize like @Ashish has done, or you can understand it conceptually. For starters, a quick answer to this question is determined by the relationship pKa + pKb = 14. You can solve for pKa and see right off the bat that the acid is a weak acid (because the pKa is high). We're not really used to seeing pKb as often, so it may be easier to understand it in terms of pKa. However, if you develop a good understanding then you can quickly see that the conjugate base is strong (low pKb), and therefore the acid (or "molecule") must be weak.

What makes a high pKa a weak acid, or a low pKa a strong acid? It is a basic understanding that is critical to chemistry. Ka is the dissociation constant of the acid (products, aka H+ and A- divided by the nondissociated acid, or HA). The relationship between pKa and Ka is pKa = -log(Ka). They mean the same thing, they tell you the strength of the acid is by how well it dissociates (as you know, strong acids completely dissociate). A high Ka means more "products" (dissociated components), and therefore a better acid. By relationship, a lower pKa also means a better acid (inversely related).

 

[Queen Beach]

PK's are just a math concept. Pk are negative logs. When you understand the math, you easily can determine acid or base strength by the size of the pk. Large pk are small k and small pk are large k. Done.

 

[Ashish]

This is a very basic question. You can either memorize like @Ashish has done, or you can understand it conceptually. For starters, a quick answer to this question is determined by the relationship pKa + pKb = 14. You can solve for pKa and see right off the bat that the acid is a weak acid (because the pKa is high). We're not really used to seeing pKb as often, so it may be easier to understand it in terms of pKa. However, if you develop a good understanding then you can quickly see that the conjugate base is strong (low pKb), and therefore the acid (or "molecule") must be weak.

What makes a high pKa a weak acid, or a low pKa a strong acid? It is a basic understanding that is critical to chemistry. Ka is the dissociation constant of the acid (products, aka H+ and A- divided by the nondissociated acid, or HA). The relationship between pKa and Ka is pKa = -log(Ka). They mean the same thing, they tell you the strength of the acid is by how well it dissociates (as you know, strong acids completely dissociate). A high Ka means more "products" (dissociated components), and therefore a better acid. By relationship, a lower pKa also means a better acid (inversely related).

I don't recommend memorization and myself did not memorize anything. Just practice a lot of them. Save memorization for Bio section 🙂

 

[tommyngu]

NH3 and HSO4- are both amphoteric, so don't eliminate HSO4- "because it is an acid". Be critical of anything you question in Kaplan. Kaplan has many mistakes.