Acid base equilibria

• 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 an acid and its conjugate base?

  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+]
• How does the concentration of hydrogen ions relate to strong acids?

  The concentration of hydrogen ions in a monoprotic strong acid will be the same as the concentration of the acid.
• How do you find [H+] from pH?

  [H+] = 1 x 10^{-pH}
• Calculate the concentration of HCl with a pH of 1.35.

  [H+] = 0.045 mol dm-3.
• What is the ionic product for water (Kw)?

  Kw = [H+(aq)][OH-(aq)]
• What is the value of Kw at 25°C?

  Kw = 1 x 10^{-14} mol² dm^{-6}
• How can you find [H+] ions if you know [OH-] ions using Kw?

  You can use the expression [H+] = Kw / [OH-].
• Why is pure water considered neutral?

  Because [H+(aq)] = [OH-(aq)].
• What is the pH of pure water at 25°C?


  The pH is 7.
• How does temperature affect the pH of pure water?

  At different temperatures, the pH of pure water changes due to the endothermic dissociation of water.
• Calculate the pH of water at 50ºC given that Kw = 5.476 x 10^{-14} mol² dm^{-6}.


  The pH is 6.6.
• How do strong bases dissociate in solution?

  Strong bases completely dissociate into their ions.
• How do you calculate the pH of a strong base like NaOH?

  First, find [H+(aq)] using the Kw expression.
• What is the dissociation expression for weak acids?

  Ka = \frac{[H+(aq)][A-(aq)]}{[HA(aq)]}
• What is the Ka for ethanoic acid?

  Ka = 1.7 x 10^{-5} mol dm^{-3}.
• How does the value of Ka relate to the strength of an acid?

  The larger the Ka, the stronger the acid.
• Write the dissociation equation for propanoic acid and its Ka expression.

  HA + H2O (l) ⇌ H3O+ (aq) + A- (aq), Ka = \frac{[H+(aq)][A-(aq)]}{[HA(aq)]}
• Calculate the concentration of propanoic acid with a pH of 3.52.

  The concentration is 6.75 x 10^{-3} mol dm^{-3}.
• What is the relationship between pKa and Ka?

  pKa = -log Ka, so Ka = 10^{-pKa}.
• What are the steps to calculate pH in neutralization reactions involving strong acids and strong bases?

  1. Calculate moles of original acid and moles of base added.
  2. Determine which is in excess.
  3. Calculate new concentration of excess H+ or OH- ions.
  4. Use pH = -log[H+] or [OH-] = Kw / [H-].
• Calculate the pH of a mixture of 35 cm³ of 0.5 mol dm-3 H2SO4 and 30 cm³ of 0.55 mol dm-3 NaOH.


  The pH is 0.55.
• What are the steps to calculate pH in neutralization reactions involving weak acids and strong bases?

  1. Calculate moles of weak acid and strong base.
  2. Determine which is in excess.
  3. Calculate new concentration of excess H+ or OH- ions.
  4. Use pH = -log[H+] or [OH-] = Kw / [OH-].
• What is the formula for calculating the concentration of hydrogen ions from hydroxide ions?

  [H^+] = \frac{K_w}{[OH^-]}
• How do you calculate the moles of HCl from its concentration and volume?

  Moles HCl = concentration × volume = 0.5 × 0.015 = 0.0075 mol
• How do you calculate the moles of Ba(OH)₂ from its concentration and volume?

  Moles Ba(OH)₂ = concentration × volume = 0.45 × 0.035 = 0.01575 mol
• What are the steps to calculate the pH of weak acid and strong base neutralizations?

  1. Work out moles of original acid.
  2. Work out moles of base added.
  3. Determine which one is in excess.
  4. Calculate new concentration of excess HA.
  5. Calculate concentration of salt formed.
  6. Use Ka to find [H⁺].
  7. Calculate pH.
• How do you calculate the pH when a buffer is made by adding sodium hydroxide to partially neutralize a weak acid?

  Calculate moles of the weak acid and NaOH, then find the excess weak acid and use the formula for pH.
• What is the principle behind buffer solutions?

  • A buffer solution resists changes in pH when small amounts of acid or alkali are added.
  • Acidic buffer: weak acid + salt of that weak acid.
  • Basic buffer: weak base + salt of that weak base.
• How does an acidic buffer work when small amounts of acid are added?

  The equilibrium shifts left, removing H⁺ ions added.
• How does a basic buffer work when small amounts of alkali are added?

  OH^- ions react with H⁺ ions to form water, shifting the equilibrium right.
• What is the formula for calculating the pH of a diluted strong acid?

  [H⁺] = \frac{[H⁺]_{old} \times V_{old}}{V_{new}}
• 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 the acid remains constant.
• What happens to the moles of buffer acid and salt when a small amount of alkali is added to a buffer?

  The moles of buffer acid decrease and moles of salt increase by the same amount.
• What is the reaction when a small amount of alkali is added to a buffer containing acetic acid?

  CH3CO2H (aq) + OH- → CH3CO2- (aq) + H2O (l)
• What occurs when a small amount of acid is added to a buffer solution?

  The moles of buffer salt decrease and moles of buffer acid increase by the same amount.
• How do you calculate the pH of a buffer solution after adding NaOH?

  1. Calculate initial moles of acid and salt.
  2. Adjust moles of acid and salt based on NaOH added.
  3. Use the new moles in the pH calculation.