Prepared byProf. Odyssa Natividad R.M. Molo

Consider a solution that contains not only a weak acid (HC2H3O2) but also a soluble salt (NaC2H3O2) of that acid.

NaC2H3O2(aq) Na+(aq) + C2H3O2-(aq)

HC2H3O2(aq) H+(aq) + C2H3O2-(aq)

What happens when salt is added? (Remember Le Chatelier’s Principle)Shifts equilibriumpH of solution increases[H+] decreases

The Common-Ion Effect“The dissociation of a weak electrolyte is

decreased by adding to the solution a strong electrolyte that has an ion common with the weak electrolyte”Plays an important role in determining the pH

of the solution & the solubility of a slightly soluble salt

Despite distinctive name, this is simply a special case of Le Chatelier’s principle

Sample Problem1) What is the

pH of a soln made by adding a 0.30 mol acetic acid & 0.30 mol sodium acetate to enough water to make 1.0L of soln?

STEPS:1)Identify the major species

in the soln & consider their acidity & basicity

2)Identify the important eqlbm rxn

3)Calculate the initial & eqlbm conc of each species that participate in the eqlbm

4)Calculate the pH from the eqlbm conc

Practice Exercise1) Calculate the pH of a solution containing

0.085M nitrous acid (Ka = 4.5 x 10-4) & 0.10M potassium nitrite. What would be its pH if no salt were present.

2) Calculate the pH of a solution containing 0.20 M acetic acid & 0.30M sodium acetate. What would be the pH if no salt were present.

Buffered Solution/BuffersSolution that resist a drastic change in pH

upon addition of small amounts of acid or base

Contain weak conjugate acid-base pairsExample:

Human blood: bicarbonate-carbonic acid buffer Normal pH: 7.35 – 7.45 Acidosis: condition when pH falls below 7.35 Alkalosis: when pH rises above 7.45 Death may result if blood pH < 6.8 or > 7.8

Composition of BuffersA buffer resist in pH because it contains both

an acidic specie to neutralize OH- ions & a basic one to neutralize H+ ions. The acidic & basic species that make up the buffer, however, must not consume each other through a neutralization rxn. These requirements are fulfilled by a weak acid-base conjugate pair.

Composition cont….Buffers are often prepared by mixing a WA or

a WB with a salt of that acid or base.Example:

HC2H3O2 - C2H3O2- buffer (NaC2H3O2 & HC2H3O2)

NH4+ - NH3 buffer (NH4Cl & NH3)

By choosing appropriate components & adjusting their relative concentration, one can buffer a solution at virtually any pH

How Buffer WorksEx: Buffer composed of weak acid (HX) &

its salt (MX)HX(aq) H+(aq) + X-(aq)

If OH- ions are added, it reacts with the acid component of the buffer to produce water & its base component (X-)

OH-(aq) + HX(aq) H2O(l) + X-(aq)Result: [HX] dec & [X-] inc

How Buffer Works cont…If H+ ions are added, it reacts with the

base component of the buffer to produce water & its acid component (HX)

H+(aq) + X-(aq) HX(aq) ORH3O+(aq) + X-(aq) H2O(l) + HX(aq)

Result: [HX] inc & [X-] dec

Buffer pHHX(aq) H+(aq) + X-(aq)

so

pH is determined by 2 factors: (1) value of Ka(2) ratio of conjugate acid-base pair

If [HX] = [X-], [H+] = Ka, pH = pKaResult: select a buffer whose acid form has a

pKa close to desired pH

2 important characteristics of buffer:

(1) its capacity & (2) its pHBuffer capacity

Is the amount of acid or base the buffer can neutralize before the pH begins to change to an appreciable degree

Depends on the amount of Acid & Base from which the buffer is made

pH of buffer depends onThe Ka for the acid & on the relative

concentration of A & B that comprise the buffer

To solve buffer pHUse the same procedure to treat the common-ion

effect OR the Henderson-Hasselbalch equation

Sample Problem: Calculate the pH of a buffer that is 0.12M lactic acid (Ka = 1.4 x 10-4) & 0.10M sodium lactate.

Practice: Calculate the pH of a buffer composed of 0.12M benzoic acid, HC7H5O2, (Ka = 6.3 x 10-5) & 0.20M sodium benzoate

Addition of SA/SB to BuffersWhen a SA is added, the H+ is consumed

by X- to produce HX; Result:[HX] inc & [X-] dec

When a SB is added, the OH- is consumed by HX to produce X-

Result: [HX] dec & [X-] inc

Steps to calculate pH1) Identify the neutralization rxn (Strong

Acid & Weak Base or Strong Base & Weak Acid)

2) Analyze and set-up condition before & after neutralization

3) Calculate pH based on what is left during equilibrium condition.

Sample ExerciseA buffer is made by adding 0.300 mol acetic

acid & 0.300 mol sodium acetate to enough water to make 1.00L soln. Calculate its pH (a) at the start; (b) after 0.020 mol KOH is added; (c) after 0.010 mol HCl is added. For comparison, what is the the pH of (d) 0.020 mol NaOH and (e) 0.010 mol HCl added in 1.00L pure water.

Practice Exercise1) What are the effects on the pH of adding (0.0060

mol HCl and (b) 0.0060 mol NaOH to 0.300L of a buffer solution that is 0.250M acetic acid (Ka = 1.8 x 10-5) & 0.560 M sodium acetate. For comparison, what is the pH of (c) 0.0060 mol HCl & (d) 0.0060 mol NaOH in 0.300L water.

2) A 1.00 L volume of buffer is made with concentrations of 0.350 M NaCHO2 (sodium formate) and 0.550M HCHO2 (Ka = 1.8 x 10-4). (a) What is the initial pH? (b) What is the pH after the addition of 0.0050 mol HCl? (assume that the volume remains 1.00L) (c) What would be the pH after the addition of 0.0050 mol NaOH to the original buffer?