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RAB-I RASHIDI UNIVERSITY CHEMISTRY LAB: TITRATION

Dr.Mohammadi. RAB-I RASHIDI UNIVERSITY CHEMISTRY LAB: TITRATION. Assumptions:. [H + ] = [A - ]. 1. No common ion:.     . K a =. . [ H +  ][ H +  ]. K a =. [H + ]=. K a c. pK a -logc. pH=. 2. pH of Weak Acids: Calculations. Calculation of pH from a :. a.

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RAB-I RASHIDI UNIVERSITY CHEMISTRY LAB: TITRATION

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  1. Dr.Mohammadi RAB-I RASHIDI UNIVERSITYCHEMISTRY LAB: TITRATION

  2. Assumptions: [H+] = [A-] 1. No common ion:  Ka =  [H+][H+] Ka = [H+]= Kac pKa-logc pH= 2 pH of Weak Acids: Calculations Calculation of pH from a: a [H+]= c Calculation of pH from Ka c 2. a is small: [HA]  c Calculation of pH from pKa pKa = -log Ka The same assumptions as above

  3. Calculation of pH from a: [OH-]= c a + OH- Calculation of pH from Kb NH4+ [OH-] Kb = NH3 Kb is an equilibrium constant of basic dissociation (concentration of H2O included) NH3 + H2O NH4+ pOH = -lg [OH-] pH = 14-pOH [OH-]= Assumptions: 1. No common ion: [OH-] = [NH4+] Kbc 2. a is small: [NH3]  c pKb-logc Calculation of pOH from pKb pOH= 2 pH = 14-pOH The same assumptions as above pH of Weak Bases: Calculations

  4. Na-acetate A- + Na+ Salt HA A- + H+ Weak acid Buffers are made of 2 components: Buffers • a weak acid • and its conjugated base (salt) Common ion: [A-] Mainly from the salt [HA] will be the dominant form of the weak acid [H+] is low Ionization is inhibited chemeq6.ppt

  5. [H+][A-] [HA] [HA] [HA] [A-] [A-] [H+]= Ka pH=pKa - log Ka = [acid] pH=pKa - log [HA]=[A-] Common ion: [salt] [salt] pH=pKa + log [acid] Buffers: Calculations Henderson-Hasselbalch equation: chemeq6.ppt

  6. 1 pH = pKa- 1 = 3.7 = 10 [Salt] [Salt] [Salt] 1 4.7 pH = pKa= 4.7 = [Acid] [Acid] [Acid] 1 10 pH = pKa+ 1 = 5.7 = 1 1:1 10:1 1:10 more salt more acid How Buffers work? Range of pH stabilization (acetic acid/Na-acetate buffer): A buffer stabilizes pH around its pKa value +1 chemeq6.ppt

  7. Weak acid H+ HA + A- HA A- H+ + Indicator • Titration of a weak acid Uknown: acetic acid, weak acid Indicator: phenolphtalein, weak acid Color in basic range No color in acidic range

  8. Titration curves of acetic acid pH 10 Equivalence point 7 5 0,5 1 Buffer effect meqv NaOH • Titration of a 0.1 M acetic acid (10 ml) with 0.1 N NaOH: NaOH excess Start: around pH 3 Buffer effect at partial neutralization Equivalence point in BASIC range Start chemeq6.ppt

  9. Strong acid H+ HA + A1- HA H+ + A2- Weak acid • Effect of strong acid on the ionization of a weak acid Common ion: [H+] Mainly from the strong acid Ionization of weak acid is inhibited by the strong acid [HA] c weak acid

  10. HCl H+ HA + A- Indicator: a weak acid, e.g. methyl orange (HA) HA A- H+ + Indicator Color in acidic range Color in basic range

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