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Author: Subject: Help pH levels

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[*] posted on 27-11-2006 at 07:52
Help pH levels

When given a chemical like NaH2PO4 how do I go about finding the acidity if this is all Im given. I am drawing a blank and I just learned this. Please advise
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[*] posted on 27-11-2006 at 08:50

If by acidity you do indeed mean the pH, then if (literally) you are just given a formula, then I don't see how you can find the pH. Other information, such as a table of Ka values would be needed. It is not clear from your post what stage of chemistry you are at, and acidity is a complex subject which can be addressed in different ways according to the depth of knowledge assumed.
If you are a complete beginner (nothing pejorative intended) then I wonder if your teacher is introducing you to the concept of acidity/basicity in the sense of the number of hydrogen ions which can be given up. In this case H2SO4 is said to have a basicity of two, NaHSO4 a basicity of one, HCl a basicity of one, H3PO4 a basicity of three, NaH2PO4 a basicity of two. Inversely, NaH2PO4 might therefore be said to have an "acidity" of one - ie it can "take back" one hydrogen atom to become H3PO4. Note that basicity and acidity are used in what seems, at least to me, a counterintuitive way. Thus H2SO4 has a basicity of two, not an acidity of two.
I may have missed the point here, but I wonder if that helps?
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International Hazard

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[*] posted on 27-11-2006 at 21:16

Look up pKa, dissociation constant (for this compound there will be two, one for each proton (H), better yet, How would you determine these constants experimentally *hint-titration*). Then make sure you know what's going on with molarity and the difference it has with normality. Then apply this to the standard HA<-->H+ + A- equilibrium. You should be good to go.

(thank heavens we are not dealing with activity, here--but you should look that up, too)

Best of luck,


-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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