Acids+Bases

• pH


  A logarithmic scale that measures the concentration of H+ ions in solution
• pH 0 is very acidic, pH 7 is neutral, pH 14 is very basic
• pH equation


  pH = -log[H+]
• Calculating pH of strong acid

  1. Assume strong acid dissociates fully

  2. Concentration of H+ = Concentration of acid
• Calculating pH of diprotic acid

  Concentration of H+ = 2 x Concentration of acid
• Calculating pH of strong base

  1. Assume strong base dissociates fully

  2. Use Kw expression to find [H+]

  3. pH = -log[H+]
• Ka


  Acid dissociation constant
• Ka is used to calculate pH of weak acids, as weak acids only dissociate slightly
• Calculating pH of weak acid


  1. Use Ka expression: Ka = [H+]^2 / [HA]

  2. Assume [HA] ≈ initial [HA]

  3. Solve for [H+], then calculate pH
• Calculating [HA] from pH and Ka


  Use Ka expression: [HA] = [H+]^2 / Ka
• Ka can also be calculated if given [HA] and pH
• Titration


  A method used to determine the concentration of an acid or base by adding a known volume of a base (or acid) of known concentration to a solution of the acid (or base) until the reaction is complete (the equivalence point)
• Titration experiment

  Acid/base of known concentration in burette

  2. Acid/base of unknown concentration in conical flask

  3. Add indicator to conical flask

  4. Add burette contents to conical flask drop by drop until indicator changes color (endpoint)

  5. Record volume added from burette
• Titration graph


  • S-shaped curve

  Vertical rise at equivalence point (when acid fully neutralized by base)

  Half-neutralization point (halfway to equivalence point, used to calculate pKa)
• Equivalence point


  The point where the acid has been fully neutralized by the base, indicated by a sharp vertical rise in the titration graph
• Half-neutralization point


  The point halfway between the start and the equivalence point, where the concentrations of acid and conjugate base are equal
• At the half-neutralization point, [HA] = [A-], so Ka = [H+]
• Indicator


  A substance that changes color at a specific pH range, used to determine the endpoint of a titration
• The indicator must change color within the vertical region of the titration graph, i.e. at the equivalence point
• Ammonium chloride breaks down into ammonium ion and chloride ions
• There are high amounts of ammonium ion and chloride ions, so equilibrium lies well over to the right
• When a base or OH- is added to the buffer


  1. The OH- reacts with the ammonium ions in solution

  2. This produces more NH3 and H2O

  3. Equilibrium shifts to the left
• When H+ ions are added to the buffer


  1. The H+ does not react with the Cl- as HCl would just dissociate back

  2. Instead the H+ reacts with the OH- that is produced from the ammonium

  3. Equilibrium shifts to the right to replace the consumed OH-
• pH of a buffer


  Calculated using the Ka value, concentration of weak acid, and concentration of salt
• Calculating pH change when adding small amount of acid/base to buffer


  1. Calculate moles of species before and after addition

  2. Calculate new concentrations accounting for volume change

  3. Use Ka expression to find new [H+] and calculate new pH
• Uses of buffers


  • Shampoo - maintains slightly acidic pH for hair
  • Washing powder - maintains optimal pH for enzymes
  • Blood - maintains constant pH of 7.4 for bodily functions
• Brønsted acid


  A species that can donate a proton
• Brønsted base


  A species that can accept a proton
• Weak acids dissociating


  1. In an equilibrium reaction, the products are formed at the same rate as the reactants are used

  2. At equilibrium, both reactants and products are present in the solution
• Acid


  Species that can donate a proton
• Base


  Species that can accept a proton
• Conjugate acid-base pair


  Two species that are different from each other by an H+ ion
• A firm increases advertising


  Demand curve shifts right
• Demand curve shifting right

  Increases the equilibrium price and quantity
• Marginal utility


  The additional utility (satisfaction) gained from the consumption of an additional product
• If you add up marginal utility for each unit you get total utility
• The acidity of an aqueous solution depends on the number of H+ (H3O+) ions in solution
• pH


  Defined as: pH = -log[H+] where [H+] is the concentration of hydrogen ions in mol dm–3
• [H+]


  Concentration of hydrogen ions, can be calculated from pH using: [H+] = 10-pH
• The pH scale is a logarithmic scale with base 10