MNU Pharmaceutical Analytical Chemistry -1 (PC101) Lecture 6 PDF

Mansoura National University Faculty of Pharmacy Pharm D-Clinical Pharmacy Program Department of Pharmaceutical Analytical Chemistry Level 1 Pharmaceutical Analytical Chemistry -1 (PC101) (Lecture 6)...

Mansoura National University Faculty of Pharmacy Pharm D-Clinical Pharmacy Program Department of Pharmaceutical Analytical Chemistry Level 1 Pharmaceutical Analytical Chemistry -1 (PC101) (Lecture 6) Prof. Dr. Jenny Jeehan Nasr Dr. Galal Magdy [email protected] Dr. Heba Samir Elama [email protected] [email protected] Pharmaceutical Analytical Chemistry Department, Faculty of Pharmacy, Mansoura University  Important definitions Oxoacid: an acid that contains oxygen (Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, e.g. HNO3 , H2SO4 , H3PO4). 2  Important definitions Acid anhydrides (Acidic oxides) Basic anhydride Nonmetal oxides that react with water to form Metal oxides that react with water to form bases. acids. (Considered air pollutants as they react with atmospheric water vapor to produce acid rain) Examples: CO2 , SO2, SO3 Examples: metal oxides (Na2O, MgO, CaO) CO2 + H2O → H2CO3 Na2O + H2O → 2NaOH SO2 + H2O → H2SO3 MgO + H2O → Mg(OH)2 SO3 + H2O → H2SO4 CaO + H2O → Ca(OH)2 4  Learning Outcomes In this lecture, we will learn about: 1. The nature of acids and bases. 2. Acid-Base Theories. 3. Acid-Base Equilibrium. 4. pH Calculations. 5  Acid-Base Equilibrium Law of Mass Action ◼ The rate of a chemical reaction is proportional to the product of multiplication of the active masses (molar concentrations) of the reacting substances. aA + bB ⇌ cC + dD [C]c[D]d Keq = [A]a[B]b Keq is a constant that is affected only by temperature & pressure.  Acid-Base Equilibrium In Acid-Base Reactions ◼ In case of weak acids: (e.g. CH3COOH) CH3COOH ⇌ CH3COO- + H+ Keq = Ka = Ionization constant of the acid or Acid dissociation constant where (for acetic acid): [CH3COO−][H+] Ka = & pKa = - logKa [CH3COOH]  Acid-Base Equilibrium In Acid-Base Reactions ◼ In case of weak bases: (e.g. NH4OH) NH4OH ⇌ NH4+ + OH- Keq = Kb = Ionization constant of the base or base dissociation constant where (for amm. hydroxide): [NH4+][OH−] Kb = & pKb = - logKb [NH4OH] N.B. Ka & Kb are calculated only for weak acids and weak bases. Why??  Acid-Base Equilibrium In Acid-Base Reactions ◼ In case of water (H2O): Pure water is a very weak electrolyte (very limited ionization). H2O ⇌ H+ + OH- [H+][OH−] Dissociation constant of water (Kw) = = [H+][OH-] [H2O] For pure water at room temperature (25°C): [H+][OH-] was experimentally found to be 1 x 10-14 M pKw = pH + pOH = 14  Acid-Base Equilibrium In Acid-Base Reactions ◼ Relation between: ◼ pKa of an acid and pKb of its conjugate base pKa + pKb = pKw = 14 e.g. CH3COOH ⇌ CH3COO- + H+ pKa (acetic acid)+ pKb (acetate) = 14 ◼ pKb of a base and pKa of its conjugate acid pKb + pKa = pKw = 14 e.g. NH4OH ⇌ NH4+ + OH- pKb (amm hydroxide)+ pKa (ammonium) = 14 ◼ Note that: small “p” means “-log” “[ ]” means “molar concentration  Learning Outcomes In this lecture, we will learn about: 1. The nature of acids and bases. 2. Acid-Base Theories. 3. Acid-Base Equilibrium. 4. pH Calculations. 11  pH Calculations In Acid-Base Reactions pH of acids and bases pH of a Strong Acid pH = - log[H+] pH of a Strong Base pOH = - log[OH-] and pH = 14 - pOH  pH Calculations In Acid-Base Reactions pH of acids and bases pH of a Weak Acid pH = ½ (pKa + pCa) {Ca is the molar concentration of the acid} pH of a Weak Base pOH = ½ (pKb + pCb) and pH = 14 - pOH {Cb is the molar concentration of the base}  pH Calculations In Acid-Base Reactions pH of salts Salt of a Strong Acid and a Strong Base pH = 7 e.g. NaCl, K2SO4 Salt of a Strong Acid and a Weak Base pH < 7 e.g. NH4Cl  pH Calculations In Acid-Base Reactions pH of salts Salt of a Weak Acid and a Strong Base pH > 7 e.g. CH3COONa Salt of a Weak Acid and a Weak Base It depends on the pKa of the acid and pKb of the base  pH Calculations In Acid-Base Reactions pH Problem Solving Diagram  The pH of a solution is 3.0. What is the [OH-]? Solution: pOH= pKw- pH pOH= 14 – 3.0 = 11 [OH-] = 1 x 10-11 M  What is the [OH-] of a solution whose [H+] = 0.001M? Solution: [H+] = 0.001 = 1 x 10-3 M −14 1  10 [OH-] = = 1 x 10-11 M 1  10 − 3  If the dissociation constant of pyridine is 1.71 x 10-9. Calculate the dissociation constant of its conjugate acid. (the conjugate acid is C6H5 NH+) Solution: Ka Kb = Kw 1.71 x 10-9 x Ka = 1 x 10-14 Ka = 1 x 10-14 /1.71 x 10-9 = 5.85x 10-6  The pKb value of strychnine is 6.0 , calculate the pKa of strychnine HCl. Solution: pKa + pKb = pKw pKa = 14 - 6 = 8  Calculate the pH of a solution of 4x10-2 M formic acid (Ka=1.7x10-4). Solution: pH = ½ (pKa + pCa) pH = - ½ log 1.7x10-4– ½ log 4x10-2 = 2.58 2013 Dr. Jenny Jeehan Nasr 21  Calculate the pH of 0.1 M ammonia solution. (pKb = 4.75). Solution: pH = pKw – ½ pKb + ½ log Cb = 14 – 2.38 + (½ log 0.1) = 11.62 – 0.50 = 11.12 2013 Dr. Jenny Jeehan Nasr 22 1. Calculate the pH of 0.06 mol/L HCl? 2. Calculate [H+] of a hydrochloric acid solution of pH 5.3 3. Calculate the pH of a 1.05x10-3 mole/L NaOH solution. 4. Calculate pH, pOH, [H+] and [OH-] of 3.2x10-4 M solution of Ba(OH)2. ✓ The nature of acids and bases. ✓ Acid-Base Theories. ✓ Acid-Base Equilibrium. ✓ pH Calculations. 24