Prepared by Prof. Odyssa Natividad R.M. Molo

Consider a solution that contains not only a weak acid (HC 2 H 3 O 2 ) but also a soluble salt (NaC 2 H 3 O 2 ) of that acid. NaC 2 H 3 O 2 (aq)  Na + (aq) + C 2 H 3 O 2 - (aq) HC 2 H 3 O 2 (aq)  H + (aq) + C 2 H 3 O 2 - (aq) What happens when salt is added? (Remember Le Chatelier’s Principle) Shifts equilibrium pH 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 Problem 1) 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 Exercise 1) Calculate the pH of a solution containing 0.085M nitrous acid (Ka = 4.5 x ) & 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/Buffers Solution that resist a drastic change in pH upon addition of small amounts of acid or base Contain weak conjugate acid-base pairs Example: 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 7.8

Composition of Buffers A 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: HC 2 H 3 O 2 - C 2 H 3 O 2 - buffer (NaC 2 H 3 O 2 & HC 2 H 3 O 2 ) NH NH 3 buffer (NH 4 Cl & NH 3 ) By choosing appropriate components & adjusting their relative concentration, one can buffer a solution at virtually any pH

How Buffer Works Ex: 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)  H 2 O(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) OR H 3 O + (aq) + X - (aq)  H 2 O(l) + HX(aq) Result: [HX] inc & [X - ] dec

Buffer pH HX(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 = pKa Result: select a buffer whose acid form has a pKa close to desired pH

2 important characteristics of buffer: (1) its capacity & (2) its pH Buffer 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 on The Ka for the acid & on the relative concentration of A & B that comprise the buffer

To solve buffer pH Use 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 ) & 0.10M sodium lactate. Practice: Calculate the pH of a buffer composed of 0.12M benzoic acid, HC 7 H 5 O 2, (Ka = 6.3 x ) & 0.20M sodium benzoate

Addition of SA/SB to Buffers When 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 pH 1) 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 Exercise A buffer is made by adding mol acetic acid & mol sodium acetate to enough water to make 1.00L soln. Calculate its pH (a) at the start; (b) after mol KOH is added; (c) after mol HCl is added. For comparison, what is the the pH of (d) mol NaOH and (e) mol HCl added in 1.00L pure water.

Practice Exercise 1) What are the effects on the pH of adding ( mol HCl and (b) mol NaOH to 0.300L of a buffer solution that is 0.250M acetic acid (Ka = 1.8 x ) & M sodium acetate. For comparison, what is the pH of (c) mol HCl & (d) mol NaOH in 0.300L water. 2) A 1.00 L volume of buffer is made with concentrations of M NaCHO 2 (sodium formate) and 0.550M HCHO 2 (Ka = 1.8 x ). (a) What is the initial pH? (b) What is the pH after the addition of mol HCl? (assume that the volume remains 1.00L) (c) What would be the pH after the addition of mol NaOH to the original buffer?