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Topic: Strong acids or bases  (Read 14466 times)

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Offline imarockstar858

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Strong acids or bases
« on: March 23, 2009, 07:16:42 PM »
How do you know whether an acid is strong or weak? (same with bases)
for example, H2PO4. Would you look at the structure.. or do you just know?

Offline Arkcon

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Re: Strong acids or bases
« Reply #1 on: March 23, 2009, 07:26:24 PM »
You can look at a table, of pKa values, if you have one handy.  But pretty much, you just memorize them.  There really aren't that many strong acids and bases, according to the Arrhenius definition.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Loyal

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Re: Strong acids or bases
« Reply #2 on: March 23, 2009, 08:31:56 PM »
How do you know whether an acid is strong or weak? (same with bases)
for example, H2PO4. Would you look at the structure.. or do you just know?

If it isn't HCl, HI, HBr, HClO4,HClO3, H2SO4, or HNO3 then more often than not it isn't a strong acid.   

A few exceptions are fluorinated sulfonic acids, but in general just about any other acid has a Ka associated with it.

And also in the case of H2PO4- that is an acid that has already given up one proton so those are weak 99.9% of the time. (I leave a .1% just in case there are acids I don't know about.)  Even Sulfuric acid is weak after the first proton.
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Offline imarockstar858

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Re: Strong acids or bases
« Reply #3 on: March 23, 2009, 08:41:04 PM »
Thank you very much!

Offline AWK

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Re: Strong acids or bases
« Reply #4 on: March 24, 2009, 02:20:06 AM »
How do you know whether an acid is strong or weak? (same with bases)
for example, H2PO4. Would you look at the structure.. or do you just know?

H2PO4 does not exist as a neutral molecule (H3PO4 or H2PO4- should be used)
AWK

Offline Borek

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Re: Strong acids or bases
« Reply #5 on: March 24, 2009, 04:22:52 AM »
in general just about any other acid has a Ka associated with it.

Every acid has a Ka.

Compare table at

http://www.chembuddy.com/?left=BATE&right=dissociation_constants
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Loyal

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Re: Strong acids or bases
« Reply #6 on: March 24, 2009, 10:50:27 PM »
in general just about any other acid has a Ka associated with it.

Every acid has a Ka.

Compare table at

http://www.chembuddy.com/?left=BATE&right=dissociation_constants
I tried going back to reword that after I saw the error, but the forum locked me out of the edit function so it was beyond my control at that point.  I had one of those moments where I yelled "You stupid idiot" to myself.   I meant to say that they have a Ka small enough that the acid does not completely dissociate completely.
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