Acids and Bases

[the prodogy]

I was wondering if there was a way to determine which acid is stronger if given a list of acids. I was given a question as follows:

Which of the following acids dissociates to the greatest extent when added to water?

A. HNO3
B. HNO2
C. H3PO4
D. H2CO3

The question basically asks, which is the strongest acid. From what can see, they are all oxyacids. The way I approached is was that I figured, the one with the most O atoms is the strongest acid, which is obviously false. Do I simply have to memorize strong acids to figure this out? What if HNO3 wasn't a choice, which would be the strongest acid then?

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[MyGoalIsMD]

My chemistry teacher gave us a list of strong acids:

HCl
HBr
HI
HNO3
H2SO4
HClO3
HClO4

We weren't ever given a question where the answer wasn't on the list, hope this helps a little.

So the answer would be HNO3 because it is the only one that is a strong acid.

 

[the prodogy]

Thanks dude. But I ran into another problem. Another question I ran into basically asked:

Which is the weakest base?
A. KOH
B. LiN(CH3)2
C. NaH
D. NaHCO3

I'm not sure how to determine which is the weakest base, or which one is a strong one. Is there a way or a list of strong bases?

 

[dedicate]

Good question. Im thinking you might need to consider the conjuate acid/base, but I would like to hear from someone who knows this well.

 

[Naijaba]

The best way to approach this problem is to see what the conjugate acids look like:

First write down the dissolution equations:
KOH -> K+ + OH- (the base is hydroxide)
LiN(CH3)2 -> Li+ + N(CH3)2- (the base is Dimethylamine without its proton)
NaH -> Na+ + H- (the base is a hydride ion)
NaHCO3 -> Na+ + HCO3- (the base is the bicarbonate ion)

Now look at the conjugate acids:
H2O
NH(CH3)2
H2
H2CO3

Now which of the above acids is STRONGEST, because it would have the WEAKEST conjugate base?
H2O - Water is a weak acid (You must know this)
NH(CH3)2 - The lone pair on the nitrogen makes this a Lewis base and it would therefore be a poor acid.
H2 - Molecular hydrogen would NOT split into its component hydrogens unless under extreme conditions (very poor acid).
H2CO3 - Carbonic acid, as its name implies, is an acid.

Since H2CO3 is really the only one of the four that fits the definition of "ACID", however weak, it must have the WEAKEST conjugate base.

 

[Rtrn2sndr]

The easiest way to tell if something is a strong or weak acid is to examine the resonance structures of the conjugate bases. The strongest acid will have a base that is stabilized by the most resonance structures.

If that doesn't narrow it down to one choice, acidity also increases with increasing electronegativity of the centrally bonded element.

Other things to look for are inductive (electron withdrawing) effects...more electronegative elements provide this. Also, as the percent-S character of the central element increases so does acidity.

Thus, a seemingly innocent, easy question is really a loaded introductory o chem question. I would imagine that if one were to diligently work with similar problems it wouldn't be necessary to go through these steps each time. However, we have so much for which to prepare that only a handful of us are skilled enough to recognize the answer immediately.