pH of Polybasic acid buffers

pH of Polybasic acid buffers
What if the mixture is not a 1:1 buffer?
Example 5: Calculate the pH of a solution made up to be
0.10 M in H 2 N(CH 2 ) 2 NH 3 Cl and 0.50 M in H 3 N(CH 2 ) 2 NH 3 Cl 2 .
x
H 2 N(CH 2 ) 2 NH + 3 + H 2 O º H 3 N(CH 2 ) 2 NH 2+ 3 + OH ! K b2 = 3.7 × 10 -7
This then is a normal type II buffer problem. Set up a table:
[H 2 N(CH 2 ) 2 NH + 3 ]
[OH – ]
[H 3 N(CH 2 ) 2 NH 2+ 3 ]
before equilibrium after equilibrium
0.10
0.0
0.10 ! x
x
0.50
0.50 + x
Fill in the
equilibrium
expression:
( x)( x)
( 0.10 − x)
− 7
0.50 +
3.5× 10 =