Question

A carbonic acid/carbonate buffer is used. Explain the action of said buffer in resisting pH changes after the addition of NaOH using relevant chemical equations and Le Chateliers Principle.

Thank you.

Answer 1

H2CO3(aq) + H2O(l) <-----------> HCO3-(aq) + H3O+(aq)

pKa1 = 6.3

when HCO3- is added the equilibrium will shift to the left and more H2CO3 will form

Henderson equation is

pH = pKa + log ( [ A-]/[HA])

when [A-] = [HCO3-]

[HA] = [H2CO3]

Let be [H2CO3] = [HCO3-] = 0.1M

pH = 6.3 + log (0.1/0.1)

pH = 6.3

Suppose 0.05 mole NaOH is added to this buffer

NaOH react with H2CO3 and consumed

2NaOH + H2CO3 --------> Na2CO3 + H2O

2mole of NaOH react with 1 mole of H2CO3

0.05 mole of NaOH react with 0.025 mole of H2CO3

Final [H2CO3] =0.1-0.025 =0.075M

Now,

pH = 6.3 + log ( [0.1]/[0.075] )

= 6.3 + 0.12

= 6.42

So, pH change is only 0.12

suppose 0.05 mole of NaOH solution is addeded to water

[OH-] = 0.05M

pOH = 1.30

pH = 14 -pOH

= 14 - 1.30 = 12.7

the pH change is from 7 to 12.7 and the difference is 5.7

In buffer solution the pH change is only 0.12 but in water the pH change is 5.7 , from this you can understand the effectiveness of buffer solution.