Saturday, July 27, 2013

Calculate pH and buffer capacity of a carbonic acid/ bicarbonate buffer solution.

Calculate pH and buffer capacity of a buffer solution composed from 0.025 M sodium bicarbonate (NaHCO3 ) and 0.00125 M carbonic acid (H2CO3) at 25 °C (Ka1 = 4.31×10-7).

Answer:
Carbonic acid has two dissociation constants. The first one produces bicarbonate and hydronium ion as described in the following chemical formula:
 
H2CO3 + H2O ↔ HCO3- + H3O+

Using the buffer equation, pH can be calculated as the following (pKa1= 6.37):


The buffer capacity, β, is calculated as the following:
pH=7.67

[H3O+]=10-pH=2.14×10-8


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