We are given the pKa(s) of Phosphoric Acid, H3PO4, a triprotic acid which can contribute up to 3 protons [H+] into solution. Each pKa corresponds to one proton dissociation.
Let us write the chemical equation for each proton dissociation.
a. pKa1 = 2.2 H3PO4(aq) + H2O(l) ⇌ H2PO4-(aq) + H3O+(aq)
b. pKa2 = 7.2 H2PO4-(aq) + H2O(l) ⇌ HPO42-(aq) + H3O+(aq)
c. pka3 = 12.7 HPO42-(aq) + H2O(l) ⇌ PO43-(aq) + H3O+(aq)
In each chemical reaction, notice that the proton, H+, is released into the solution, forming H3O+ reducing one proton from the acid each time.
For phosphoric acid, H3PO4, the pKas are:
pKa1 = 2.2
pKa2 = 7.2
pKa = 12.7
a) Write the reaction associated with K a2.
b) Write Ka2 in terms of the concentration of the reactants and products in the reaction above?
c) To make a buffer that is close to neutral pH, how would you use some of the following. Be precise in terms of what you would use and how much of each.
0.001 M HCl, 1 M HCl, 10 M HCl
0.001 M H3PO4, 1 M H3PO4
0.001 M NH3, 1 M NH3
Solids: Fe, Na, NaCl, NaH, NaOH
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