Acid-Base Question

[henryb]

Hey Guys,
Have been trying to figure this question out. Any help appreciated!

Indicate the ionic species that predominate at pH 8
I: NH4(+) II: NH3 III: H3PO4 IV: H2PO4(-) V: HPO4(2-) VI: PO4(3-)
a) I & IV
b) I & III
c) II & V
d) I & V
e) II & VI

How can you tell just by knowing the pH? I'm confused.
How would you work this out if it said at pH 11?

Thanks

[Borek]

You need pKa (or pKb) values for ammonia and phosphoric acid. Then you have to use Henderson-Hasselbalch equation.

http://www.chembuddy.com/?left=pH-ca...on-hasselbalch

[henryb]

How do you get an answer for this using the Henderson-Hasselbalch equation?

[RobJim]

pH = pKa + log([A-]/[HA])

You know pH, you know pKa, the only thing left is the ratio of dissociated acid with nondissociated acid. Run it once for ammonia and once for the phosphoric acid. Once you know what the log term is, you can determine whether the ratio is greater than or less than one, which tells you whether the dissociated form or the undissociated form of the species predominates.

[henryb]

Hey,

Thanks a lot Rob! I got it now