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NS Chem QBank 2: Solutions, Acids, and Electro #16
Posted: Fri Nov 22, 2019 8:04 pm
The question is asking to identify which compound contains a weak Bronsted-Lowry base. The answer is D potassium acetate. The acetate anion is the conjugate base of the weak acid acetic acid. Don't weak acids have relatively strong conjugate bases making answer D not the correct answer?
I've noticed some inconsistency in the answers to some NextStep questions regarding this topic. Maybe I'm missing something, so I'm going to review this concept right now but from what I understand strong acids contain weak conjugate bases and strong bases contain weak conjugate acids.
Thank you in advance for the help.
Re: NS Chem QBank 2: Solutions, Acids, and Electro #16
Posted: Sat Nov 23, 2019 2:18 pm
There is essentially a mismatch between a common rule of thumb given ("Weak acids have strong conjugate basis") and uh... well, reality. Weaker acids to tend to have stronger conjugate bases, and stronger bases tend to have weaker conjugate acids. But the term 'weak acid' and 'weak base' refers to any species that does not completely (or virtually completely) dissociate and therefore has a set of meaningful pKa/Ka/Kb/pKb values. Since these values are all derived from the reaction equilibrium of incomplete dissociation, they go hand-in-hand with the definition of weak acid and weak base.
Acetic acid therefore, while a weak organic acid, does indeed dissociate into another 'weak' organic base. This base is relatively strong, but by it's nature, it certainly does not fully abstract one proton per molecule of acetate - which would be the requirement for it to be considered a 'strong' B-L base. If it did so, that would mean every molecule would become acetic acid - and then acetic so-called-acid would in turn not be dissociating into acetate and hydronium ion, making it not an acid at all.
I do agree that some of our older materials treat this topic and common point of confusion a little bit too loosely in the explanations, although our answer-keys are generally correct.