12 AP Chemistry Buffers

What is a Buffer? A buffer solution is one, which can absorb additions of acids or bases with only slight changes in pH values. It is a combination of a weak acid or weak base with one of its salts. Ex. NH3 with NH4Cl, or HNO2 with KNO2 The acid portion of the buffer reacts with any base which is added (and vice versa with base) The capacity of the buffer would depend on the concentrations of the conjugate acid/base pair of the buffer.

Consider the following system: Acetate/acetic acid buffer When OH- is added: When H3O+ is added:

Calculating the pH of a buffer Assume that both [HA] the acid, and [A-] the conjugate base, are the same as the initial concentrations for approximate value.

Example Calculate the pH of 500mL of a buffer solution composed of 0.5M formic acid (HCO 2 H) and 0.70M sodium formate (NaHCO 2 ). The Ka of formic acid is 1.8x10 -4 Since the volume is the same, we could use mols instead of [M]

As H 3 O + is added to a buffer, the ions react with the base, reducing the # moles of the base portion of the buffer and increasing the # moles of acid portion of the buffer by the same amount. We can determine the approximate [H 3 O + ] by using the following formula:

Example EXAMPLE: Calculate the pH of a 500mL buffer solution composed of 0.50M formic acid and 0.70M sodium formate after adding 10.0mL of 1.0M HCl.

Henderson-Hasselbach Equation EXAMPLE: How can we make a buffer solution that has a pH of 4.30? ([H 3 O+] = 5.0 x ) Ka of acetic acid is 1.8 x