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### Topic: Ka Values and Acid Strength  (Read 214361 times)

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#### Sette

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##### Ka Values and Acid Strength
« on: May 10, 2005, 08:44:58 AM »
Hey this question stumped me.
I have to List the below with the strongest acid being 1 and the weakest acid being 4, i had a go with what i know just about the names, but the Ka values throw me.

Hydrofluoric acid - KHF = 3.5 x 10-4
Sulfuric acid - Kh2so4 = 1.2 x 10-2
Acetic acid - Kch3cooh = 1.8 x 10-5
Phosphoric acid- Kh3po4 = 7.5 x 10-3

Basically i went with Phos, Sulf, Hydro, Acetic. Strongest to weakest.
Any help appreciated thx.

#### AWK

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##### Re:Ka Values and Acid Strength
« Reply #1 on: May 10, 2005, 10:33:57 AM »
Sort by power of 10; -2 is strongest acid, -5 is a weakest one
AWK

#### paperclip

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##### Re:Ka Values and Acid Strength
« Reply #2 on: August 30, 2005, 01:40:21 AM »
Ka values are good indicators of the ability to ionise. A weak acid hardly dissolves by ionising while a strong acid completely dissolves. However, even a weak acid can have a lower pH value in solution provided we add LOTS AND LOTS OF IT! Like glacial acetic acid. That is why the pKa is so useful, since the ratio of H+ and A- is always dependant on the denominator of HA, which I think doesn't have to even be dissociated so as to be remain in solution!

That's why I think that weak acids can be EVEN WORSE than strong acids in insufficient amounts.

#### Blueshawk

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##### Re:Ka Values and Acid Strength
« Reply #3 on: August 30, 2005, 10:20:54 AM »
Hey this question stumped me.
I have to List the below with the strongest acid being 1 and the weakest acid being 4, i had a go with what i know just about the names, but the Ka values throw me.

Hydrofluoric acid - KHF = 3.5 x 10-4
Sulfuric acid - Kh2so4 = 1.2 x 10-2
Acetic acid - Kch3cooh = 1.8 x 10-5
Phosphoric acid- Kh3po4 = 7.5 x 10-3

Basically i went with Phos, Sulf, Hydro, Acetic. Strongest to weakest.
Any help appreciated thx.

When looking at their Ka's..

The smaller the number the weaker the acid.

suchas:   1.8 x 10-5  is a smaller number than 3.5 x 10-4 , which is smaller than  7.5 x 10-3, which is smaller than  1.2 x 10-2.

Thus the strongest to the weakest is:

Kh2so4  >  Kh3po4  >  KHF  >  Kch3cooh.

Does this help with understanding the Ka's ??

You can also guess acid strength by looking at the anions of the acids.

The more stable the cooresponding anion, the stronger the acid.

#### sdekivit

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##### Re:Ka Values and Acid Strength
« Reply #4 on: August 30, 2005, 11:44:57 AM »
Ka is expressed as followed, for the equilibrium HA + H2O <--> A(-) + H3O(+)

Ka = [A(-)][H3O(+)]/[HA]

this gives the ratio ionized / not ionized. Thus when Ka is high, the denominator ionized is greater and thus the stronger the acid.