[imarockstar858]

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?

[Arkcon]

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.

[Loyal]

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 K_a 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.

[imarockstar858]

Thank you very much!

[AWK]

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)

[Borek]

in general just about any other acid has a K_a associated with it.

Every acid has a K_a.

Compare table at

http://www.chembuddy.com/?left=BATE&right=dissociation_constants

[Loyal]

in general just about any other acid has a K_a associated with it.

Every acid has a K_a.

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 K_a small enough that the acid does not completely dissociate completely.