where

# Buffers (Acid & Base)

Submitted by jacquelinesweet16 on Wed, 04/06/2011 - 21:39

How much does the pH shift when 1 ml of 0.1 M NaOH is added to 10 mL of H20 (express your answer in + or - pH units.
How about when 1 mL of 0.1 M NaOH is added to 10 mL of buffer?

How much does the pH shift when 1 mL of 0.1 M HCl is added to 10 mL of H20?
When 1 mL of 0.1 M HCl is added to 10 Ml of buffer?

Since strong acids and strong bases conpletely ionize the NaOH and HCl added to water is pretty easy to do.  Let's do those first.

NaOH added to water (initial pH=7)
[OH-] = .001L x .1 mol/L / .011 L   =9.09E-3
pOH = 2.04  so pH = 11.96  (a change of 4.96 pH units)

[H+] = .001L x .1 mol/L / .011 L   =9.09E-3
pH = 2.04  (a change of 4.96 pH units)

As you can see, a small amount of a strong acid or base can have a huge impact on the pH of a solution.

A buffer is a solution of an acid and its conjugate base that minimizes the effect of adding an acid or base to the mixture.  If an acid is added, the base part of the buffer neutralizes the excess acid, while if a base is added, the acid neutralizes the additional base.  The most effective buffers are close to be equi-molar in the acid and its conjugate base.

So in the case of the 1mL of .1M NaOH added to the buffer (let's assume that it's 1M in both the acid and base and has a Ka=1x10-7)

Since the Ka expression can be solved for [H+] and assuming the concentrations of the acid and its conjugate base are 1M.
[H+] = Ka [HA]/[A-] =  1E-7 x 1M /1M
The [H+] will be 1E-7 with a pH of 7 for the buffer alone.

Adding the acid will decrease the base portion of the buffer by .0001M while increasing the acid portion by the same amount.  This will result in new concentrations for both the acid and base portion of the buffer.

Plugging those into the equation for [H+] yields
[H+] = Ka [HA]/[A-] = 1E-7 x 1.0001M / .9999M = 1.002E-7
or a pH = 6.9999 (or a pH change of .0001 pH units)

The effect of adding the same amount of base would have the same consequences, increasing the pH by .0001 pH units.