Homework Help: AP CHEMISTRY

Posted by NIKITA on Wednesday, February 15, 2012 at 5:05pm.

Phosphoric acid is triprotic acid (three ionizable hydrogens). The values of its stepwise ionization constants are Ka1= 7.5E-5, Ka2= 6.2E-8, and Ka3= 4.2E-13

1) Write the chemical equation for the first ionization reaction of phosphoric acid with water

2)Write the equilibrium constant expression (Ka1) for this reaction.

3)What would be the pH of a solution when [H3PO4]= [H2PO4-]?
(Note: pH= -log[H3O+])

AP CHEMISTRY - DrBob222, Wednesday, February 15, 2012 at 5:11pm
What is your trouble with this? What do you not understand?
AP CHEMISTRY - NIKITA, Wednesday, February 15, 2012 at 5:14pm
I don't understand how to do the first one.
I really don't know how to do reactions
AP CHEMISTRY - DrBob222, Wednesday, February 15, 2012 at 5:27pm
You know acid + water ionizes to give H3O^+ so take the H3O^+ out and see what's left.
H3PO4 + H2O ==> H3O^+ + H2PO4^-
Then Ka1 = (H^+)(H2PO4^-)/(H3PO4)

For the pH.
The pH when (H2PO4^-) = (H3PO4). Note one is in the numerator, the other is in the denominator, they cancel and you are left with Ka1 = (H^+), convert that to pH.
AP CHEMISTRY - george, Wednesday, February 15, 2012 at 5:35pm
thanx very much!!!!!
AP CHEMISTRY - DrBob222, Wednesday, February 15, 2012 at 5:51pm
You're welcome. It helps us if you DON'T change screen names.

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