Acids, Bases and Buffers

• What is the Bronsted-Lowry definition of an acid?

  A Bronsted-Lowry acid is a substance that can donate a proton.
• What is the Bronsted-Lowry definition of a base?

  A Bronsted-Lowry base is a substance that can accept a proton.
• In the reaction HCl (g) + H2O (l) → H3O+ (aq) + Cl- (aq), what are the roles of HCl and H2O?

  HCl is the acid and H2O is the base.
• What is the relationship between acids and their conjugate bases?

  • Each acid is linked to a conjugate base on the other side of the equation.
• What is the formula to calculate pH?

  pH = - log [H+]
• What does [H+] represent in the pH formula?

  [H+] is the concentration of hydrogen ions in the solution.
• How do strong acids behave in solution?

  Strong acids completely dissociate in solution.
• How should pH values be presented in exams?

  Always give pH values to 2 decimal places.
• How does the concentration of [H+] relate to [OH-] in pure water?

  In pure water, [H+(aq)] = [OH-(aq)].
• What happens to the pH of pure water when the temperature increases?

  The pH decreases as the concentration of H+ ions increases.
• How do strong bases behave in solution?

  Strong bases completely dissociate into their ions.
• What does a larger Ka value indicate about an acid?

  A larger Ka indicates a stronger acid.
• How do you calculate the pH of a weak acid?

  Use the Ka expression and assumptions about equilibrium concentrations.
• What is the relationship between pKa and Ka?

  • pKa = -log Ka
  • Ka = 10^-pKa
• What are the steps to calculate pH in neutralization reactions?

  1. Calculate moles of acid and base.
  2. Determine which is in excess.
  3. Calculate new concentration of excess ions.
  4. Use pH formula to find pH.
• How do you calculate the pH of a mixture of strong acid and strong base?

  Find moles of acid and base, determine excess, and calculate pH.
• What are the steps to calculate the pH of weak acid and strong base neutralizations?

  • Work out moles of original acid
  • Work out moles of base added
  • Determine which one is in excess
  • Calculate new concentration of excess HA
  • Calculate concentration of salt formed
  • Rearrange Ka = [H+] [A-] / [HA] to find [H+]
  • pH = -log [H+]
• What are the steps to calculate the pH of a buffer solution?

  • Calculate moles of weak acid and salt
  • Use the formula [H+] = Ka x [HA] / [A-]
  • Calculate pH = -log [H+]
• What is the formula for calculating pH?

  pH = -log [H+]
• What is the process for calculating the pH of buffer solutions?

  • Use the weak acid dissociation expression
  • Assume [A-] concentration is due to added salt only
  • Rearrange to find [H+]
  • Assume initial concentration of acid remains constant
• What happens to the moles of buffer acid and salt when a small amount of alkali is added?

  The moles of buffer acid decrease and moles of salt increase by the same amount.
• What happens to the pH of a buffer solution when it is diluted with water?

  • The pH remains unchanged
  • The ratio of [HA]/[A-] stays constant
  • Both concentrations are diluted equally
• What are the four main types of titration curves?

  1. Strong acid and strong base
  2. Weak acid and strong base
  3. Strong acid and weak base
  4. Weak acid and weak base
• What is the pH at the equivalence point for a strong acid and strong base titration?

  pH = 7
• What is the general shape of the titration curve for a strong acid and strong base?

  Long steep part from around pH 3 to 9
• What is the pH at the equivalence point for a weak acid and strong base titration?

  pH > 7
• What occurs at half neutralization volume for weak acids?

  At ½ the neutralization volume, [HA] = [A-]
• What is the relationship between Ka and pKa at half neutralization volume?

  Ka = [H+] and pKa = pH
• What are the steps to construct a pH curve?

  1. Calibrate the pH meter
  2. Transfer acid to a conical flask
  3. Measure initial pH
  4. Add alkali in small amounts
  5. Stir to equalize pH
  6. Record pH
  7. Repeat with smaller volumes near endpoint
  8. Add until alkali is in excess
• What is the pH range for phenolphthalein indicator?

  Phenolphthalein changes from colourless in acid to pink in alkali.
• What is the pH range for methyl orange indicator?

  Methyl orange changes from red in acid to yellow in alkali.
• When should methyl orange be used as an indicator?

  Use methyl orange with strong acids but not weak acids.
• When should phenolphthalein be used as an indicator?

  Use phenolphthalein with strong bases but not weak bases.
• How do indicators work in titrations?

  • Indicators are weak acids
  • They change color at different pH levels
  • The color change indicates the endpoint of titration
  • The correct indicator matches the equivalence point of the titration