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pH – Strong Acids & Bases

pH – Strong Acids & Bases. Chapter 8.4. Concentration. Reminder: 1 mole of a substance = the mass of the substance Example: sodium chloride Na = 23.0 amu  1 mole = 23.0 grams Cl = 35.5 amu  1 mole = 35.5 grams 1 mole of NaCl = 23.0 + 35.5 = 58.5 grams. Concentration.

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pH – Strong Acids & Bases

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  1. pH – Strong Acids & Bases Chapter 8.4

  2. Concentration • Reminder: • 1 mole of a substance = the mass of the substance • Example: sodium chloride • Na = 23.0 amu  1 mole = 23.0 grams • Cl = 35.5 amu  1 mole = 35.5 grams • 1 mole of NaCl = 23.0 + 35.5 = 58.5 grams

  3. Concentration • To compare the number of solute particles in solutions, chemists often use moles to measure concentration. • Molarity: moles of a solute per liter of solution or Mole L One liter of 1M NaCl solution contains 58.5 grams of NaCl.

  4. The pH Scale • pH scale: 0 to 14 • Describes concentration of hydronium ions (H3O+) • pH 7 is neutral • Acids: pH < 7 (0-6) • Bases: pH > 7 (8-14)

  5. The pH Scale • The pH scale classifies solutions as acids or bases.

  6. The pH Scale • Pure water ionizes slightly • Arrow pointing left is longer than pointing right because: • The reaction favors the reactant • Water contains many more water molecules than ions.

  7. The pH Scale • Pure water is neutral • Small but equal concentrations of: • Hydronium ions [H3O+] • Hydroxide ions [OH–] • At 25°C both [H3O+] and [OH–] is 1.0 × 10–7 M (in pure water) • pH is related to the exponent of the molarity of [H3O+] • Pure water has a pH of 7.

  8. The pH Scale • Concentrations of H3O+ and of OH– behave like they’re on a teeter totter • Adding acid to water increases [H3O+] and decreases [OH–] • Example: 0.1M Hydrochloric acid solution • Concentration of H3O+ is 1.0 × 10–1 M • Concentration of OH– is 1.0 × 10–13 M • pH is related to the exponent of the molarity of H3O+ • pH = 1

  9. Strong Acids and Bases • When acids and bases form ions in solution: • Sometimes involves complete dissociation • Strong Acid or Base • Sometimes only partially ionize • Weak Acid or Base

  10. Strong Acids and Bases • Quick reminder: • When reactions go to completion: show with “” • When reactions reach equilibrium : show with “ ” • Strong Acids and Bases • Formation of ions from the solute goes to completion. • Examples: • Hydrochloric Acid is a strong acid - total ionization: HCl + H2O  H3O+ + Cl– • Sodium Hydroxide is a strong base – total dissociation NaOH  Na+ + OH–

  11. Weak Acids & Bases • Weak Acids and Bases • Ionize or dissociate only partially in water. • Most of the hydrogens and hydroxides continue to hang on • Only a few go off on their own (dissociate) • A solution of acetic acid, CH3COOH, and water can be described by the following equation: • Equilibrium favors reactants over products • few ions form in solution.

  12. Strong and Weak • Two acids of same molarity (concentration): Weak acid: • Forms fewer ions (dissociates less) • Most of the weak acids still hanging on to their protons • Lower [H3O+] gives higher pH (closer to neutral) Strong acid: • Forms lots of ions (dissociates almost completely) • Most of the strong acids given up all of their protons • Higher [H3O+] gives lower pH (farther from neutral)

  13. Strong and Weak • Concentration and strength both affect pH. • Concentration: molarity (amount of solute dissolved in a given amount of solution). • Strength: solute’s tendency to form ions in water • Strong : total dissociation • Weak: partial dissociation.

  14. Strong Acids & Bases • There are only 6 common strong acids: • HCl - hydrochloric acid • HBr - hydrobromic acid • HI - hydroiodic acid • HNO3 - nitric acid • H2SO4 - sulfuric acid • HClO4 - perchloric acid • Common Strong bases come from the hydroxides of metals in Group 1A & 2A. • Most common are: • LiOH - lithium hydroxide • NaOH - sodium hydroxide • KOH - potassium hydroxide • RbOH - rubidium hydroxide • CsOH - cesium hydroxide • Ca(OH)2 - calcium hydroxide • Sr(OH)2 - strontium hydroxide • Ba(OH)2 - barium hydroxide

  15. Buffers • Buffer : a solution that is resistant to large changes in pH. • Weak acids and bases can be used to make buffers. • Buffers can be prepared by mixing: • a weak acid and its salt or • a weak base and its salt. • Critical for human body to maintain stable pH • Many cellular reactions very sensitive to pH • Many cellular reactions create excess hydronium ions

  16. Carbon Dioxide & the Body • CO2 dissolved in blood forms carbonic acid - a weak acid. [CO2 + H2O H2CO3] • Carbonic acid and bicarbonate ions form an important pH buffer [H2CO3 HCO3- + H+] • Carbon dioxide is exhaled, shifting the equilibrium: CO2 + H2O H2CO3 HCO3- + H+]

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