Buffer laa
Bongiorno
(scusi il italiano)
Strikes me that to do a buffer calculation you need to know the concentration of the sodium phosphate (or whatever the negative ion is in the buffer) - you cannot solve for two unknowns in one equation
pKa = pH + log[A-]/[HA]
or does phosphoric acid self-buffer?
H3A = H2A- + H+
H2A- = HA(2-) + H+
HA(2-) = A(3-) + H+
No, it can't be a self buffer with a pH of 7.3 that's just silly - how could it (phosphoric acid) be basic?
Unless this is the value that the question is asking for - the value of the Na2HPO4 concentration... hmmm
ok then taking [H3A] as 0.2M
pH = 7.3 = -log(10)[H+]
therefore H+ ion concentration = 5.011exp-8
ka = [H+] x [H2A-]/[H3A]
but as ka is unknown and [H2A-] also then it can't be done without the pKa values
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