Ch 17

BLB

Additional Aspects of Acid / Base Equilibria

Use a weak acid HC2H3O2 along with a soluble salt of the acid like NaC2H3O2

HC2H3O2 – NaC2H3O2 -

HC2H3O2 NaC2H3O2 Equilibrium shift?

17.1 The Common Ion Effec t

“The extent of ionization of a weak electrolyte is decreased by adding to the solution a strong electrolyte that has an ion in common with the weak electrolyte.”

17.1 The Common Ion Effec t

What is the pH of a solution made by adding 0.30 mol of acetic acid (HC2H3O2) and 0.30 mol NaC2H3O2 to enough H2O to make a 1.0L solution. Ka = 1.8 x10-5

Calculating pH when a common ion is involved

Calculate the pH of a solution containing 0.085 M nitrous acid (HNO2; Ka = 4.5 x 10-4) and 0.10 potassium nitrite, KNO2

Practice

Calculate F- and pH of a solution that is 0.20 M HF and 0.10M HCl. Ka = 6.8 x 10-4

Practice

Calculate the formate ion concentration and pH of a solution that is 0.050M formic acid (HCHO2; Ka = 1.8 x 10-4) and 0.10M HNO3

Practice

Solutions with weak conjugate acid-base pairs can resist drastic changes in pH upon the addition of a small amounts of strong acid or base.

BloodSeawater

17.2 Buffered Solutions

Buffers resist pH changes because it contains both an acidic species to neutralize OH- ions and a basic species to eliminate the H+ ions

However, the acidic and basic species that make up the buffer must not neutralize each other.

Buffers are often made by mixing a weak acid or weak base with a salt of that acid or base

17.2 Buffered Solutions

An HC2H3O2 – C2H3O2- buffer can be made by adding NaC2H3O2 to a solution of HC2H3O2

NH4+-NH3 buffer made by adding NH4Cl to a

solution of NH3

By choosing appropriate components and adjusting their relative concentrations, we can buffer a solution at any pH

17.2 Buffered Solutions

Consider a buffer made of a weak acid (HX) and one of its salts (MX). Acid dissociation equilibrium in the buffered solution involves both the acid and its conjugate base:

17.2 Buffered Solutions

So the pH is determined by the Ka for the weak acid and the ratio of the concentrations for the conjugate acid-base pairs

17.2 Buffered Solutions

Because conj a-b pairs share a common ion, we can use the same process to calculate pH of buffers as we did for the common ion effect

We can also take an alternative approach called the Henderson-Hasselbach equation

Calculating the pH of a buffer

In doing equilibrium calculations, we can normally neglect the amounts of acid/base of the buffer that ionize

This allows us to use the Henderson-Hasselbach equation

Calculating the pH of a buffer

pH = pKa + log[base][acid]

What is the pH of a buffer that is 0.12M lactic acid (HC3H5O3) and 0.10M sodium lactate. For lactic acid, Ka = 1.4 x 10-4

Calculating the pH of a buffer

Calculate the pH of a buffer composed of 0.12M benzoic acid and 0.20M sodium benzoate. Ka = 6.3 x 10-5

Calculating the pH of a buffer

How many moles of NH4Cl must be added to 2.0 L of 0.10M NH3 to form a buffer whose pH is 9.00? Assume the addition of NH4Cl does not change the volume. Kb = 1.8 x 10-5

Calculating the pH of a buffer

The 2 important characteristics of a buffer are its capacitypH range

Buffer capacity is the amount of acid or base the buffer can neutralize before the pH changes to an appreciable degree

Buffer capacity depends on the amount of acid and base from which it is made

Buffer Capacity and pH Range

A 1-L solution that is 1M HC2H3O2 and 1M NaC2H3O2 has the same pH as a 1-L solution that is 0.1M HC2H3O2 and 0.1M NaC2H3O2 due to the ratio in the Henderson-Hasselbach eqn

The first has a greater buffering capacity because it has more of the conj acid/base pairs

Buffer Capacity and pH Range

The pH range of any buffer is the pH range over which the buffer acts effectively

Buffers most effectively resist a change in pH in either direction when the concentration of the weak acid and conj. base are about the same

When conc of weak acid = con conj base then…The pH = pKa

This gives the optimal pH of any buffer

Buffer Capacity and pH Range

Therefore, we usually try to select a buffer whose acid form has a pKa close to the desired pH

Buffers usually have a usable range within ±1pH unit of pKa

Buffer Capacity and pH Range

Q: What is the optimal pH buffered by a solution containing HC2H3O2 and NaC2H3O2? The Ka for HC2H3O2 = 1.8 x 10-5

Buffer Capacity and pH Range