The weak base commonly known as Tris [ tris(hydroxymethyl)aminomethane], has pKa
ID: 1063283 • Letter: T
Question
The weak base commonly known as Tris [ tris(hydroxymethyl)aminomethane], has pKa = 8.3 at 20C and is commonly used in biochemical buffer solutions.
a) At what pH would you expect Tris to act as a buffer in a solution that has equal molar concentrations of base and its conjugated acid?
b) What is the pH after addition of 4.0mmol NaOH to 100 ml of buffer with pH found in part a) and total concentration (base and conjugate acid) of 300 mM?
c) What is the pH after addition of 6.0mmol HNO3 to the same buffer as in part b)?
Explanation / Answer
(a) For equimolar concentrations of acid and its conjugate base,
pH = pKa = 8.3
(b)
Initial mmol of acid = 300*100/1000 = 30 mmol
initial mmol of salt = 30mmol
mmol of NaOH added = 4.0 mmol
Final mmol of acid = 30 - 4.0 = 26.0 mmol
dinal mmol of salt = 30 + 4.0 = 34.0
pH = pKa + Log[salt]/[acid]
pH = 8.3 + Log[34.0/26.0]
pH = 8.42
(c)
Initial mmol of acid = 26.0 mmol
initial mmol of salt = 34.0 mmol
mmol of HNO3 added = 6.0 mmol
Final mmol of acid = 32.0 mmol
final mmol of salt = 28.0 mmol
pH = 8.3 + Log(28.0/32.0)
pH = 8.24
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