Respuesta :
Answer:
a) Ka2 is the most important
b) The pH of the buffer is 7.41
Explanation:
Step 1: Data given
Molarity of NaH2PO4 = 0.21 M
Molarity of Na2HPO4 = 0.34 M
a) Which Ka value is most important to this buffer?
Phosphoric acid will ionize in aqueous solution according to the following equilibrium reactions
H3PO4(aq) + H2O(l) → H2PO4-(aq) + H3O+(aq) Ka1 = 7.2*10^-3
H2PO4-(aq) + H2O(l) → HPO42-(aq) + H3O+(aq) Ka2 = 6.3 *10^-8
HPO42-(aq) + H2O(l) → PO43-(aq) + H3O+(aq) Ka3 = 4.2 * 10^-13
Ka2 is the most important constant, since it involves the equilibrium between the 2 phosporic acid salt species you have. (H2PO4- and HPO42-)
b) What is the buffer pH?
H2PO4-(aq) + H2O(l) → HPO42-(aq) + H3O+(aq)
H2PO4-(aq) → HPO42-(aq) + H+
pH = pKa + log [conjugate base]/[acid]
pH = -log(6.3 * 10^-8) + log(0.34/0.21)
pH = 7.2 + 0.21
pH = 7.41
The pH of the buffer is 7.41
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