which of the following has the weaker conjugate base?

[lwergod]

H-Cl

or

NH4+

the answer is supposedly H-Cl... However, I rolled with NH4+ thinking that no charge=more stable=weaker conj. base. I know if both conj. bases had the same charge, Cl- would be weaker since bigger/more e-neg. atoms stabilize the negative charge better. thoughts?

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[rahrahb]

H-Cl

or

NH4+

the answer is supposedly H-Cl... However, I rolled with NH4+ thinking that no charge=more stable=weaker conj. base. I know if both conj. bases had the same charge, Cl- would be weaker since bigger/more e-neg. atoms stabilize the negative charge better. thoughts?

HCL is the correct answer. HCL is the stronger acid but has the weaker conjugate bases. When HCL is in an aqueous solution, the two atoms dissociate completely into ions. If the charge of an acid is neutral then they will dissociate. If the charge of an acid is not neutral then they can either partly dissociate or not dissociate at all.

I hope that helps!

 

[Veni vidi vicii]

H-Cl

or

NH4+

the answer is supposedly H-Cl... However, I rolled with NH4+ thinking that no charge=more stable=weaker conj. base. I know if both conj. bases had the same charge, Cl- would be weaker since bigger/more e-neg. atoms stabilize the negative charge better. thoughts?

That's a tough one because of the reasoning you provided. I think in cases like these it's important to know the 7 strong acids, and HCl is one of them so would have a weak conjugate base. NH4+ is weakly acidic because it doesn't dissociate all of its hydrogens (turns into NH3)while HCl has 100% dissociation, thus NH4+ is weakly acidic, and would have a stronger conjugate base than HCl.

Using the mnuenomic CARDIO (Charge, atom, resonance,etc) always helps, but remember the basics (@rahrahb stated) a strong acid dissociates completely, so just based off of that you know NH4+ is a weak acid, thus having a stronger conjugate base than HCl which is a strong acid because it completely dissociates.

 

[Daniel Schruth]

This is a tricky concept. When comparing the stability of two conjugate bases, where one is neutral and the other bears a negative charge (i.e. NH3 and Cl-), you cannot just assume that the neutral one will be more stable. Though it is true that when comparing the same atom with or without a negative charge (i.e. NH3 vs. NH2-), the one with the negative charge will be less stable, you cannot make these comparisons across different atoms. So, to answer this question, you must have knowledge of pKa's or remember that HCl is one of your strong acids. Therefore, when HCl dissociates, it will form a very stable, weak conjugate base.