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Buffer or Common Ion problems. Identify the weak acid or base in the problem. There has to be one or the other to create a buffer. Write the ionization reaction for the weak acid or weak base. Do you recognize the common ion? It is a part of the salt. Write the equilibrium expression.

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buffer or common ion problems
Buffer or Common Ion problems.
  • Identify the weak acid or base in the problem. There has to be one or the other to create a buffer.
  • Write the ionization reaction for the weak acid or weak base. Do you recognize the common ion? It is a part of the salt.
  • Write the equilibrium expression.
  • If no volume are involved assigned the Initial concentrations to the two species in the equilibrium reaction that were given. What is the relationship in the Changes? What will be the relative Equilibrium concentrations.
buffer or common ion problems1
Buffer or Common Ion problems.
  • If Volumes are given the initial concentrations will change when the solutions are added. You must calculate the changes using M1V1 = M2V2.
  • Test for the need to use the quadratic equation. Assign the Equilibrium concentrations in the equilibrium expression and solve for the unknown.
  • Find the information requested in the problem. Most likely a pH value.
addition of small amounts of acid or base to a buffer
Addition of small amounts of acid or base to a buffer.
  • The addition, first result in a reaction with the added substance and one of the conjugate pair in the equilibrium. If you add an acid it will react with the base of the pair. This will reduce the moles of base and increase the moles of the acid in the pair. If you add a base it will react with the acid in the pair. This will reduce the moles of acid and increase the moles of base. The volume will also change because you have added a new solution.
addition of small amounts of acid or base to a buffer1
Addition of small amounts of acid or base to a buffer.
  • First decide which reaction will take place.
  • You must now determine the number of moles of each of the three key parts of this reaction. The moles of the conjugate pair for the buffer and the moles of the substance added. M = mol/L or M = mmol/mL.
  • Make the stoichiometric adjustments to the numbers of moles. Two should decrease and one increase. The added substance should be the limiting reagent and should be completely consumed.
  • Now calculate the molarity of the conjugate pair from the equilibrium.
  • Using these new concentration calculate a new pH.
titration problems
Titration problems
  • A titration is first a reaction. The types of reactions we are focused on are acid-base reactions. The result of these reactions is the formation of a salt. In cases where the base has a hydroxide you will also get water. However, it is the ions of salt that can effect the pH of the solution when all the acid and base have been neutralized and the pH will vary from a neutral seven. You must also identify the ion that effects the pH. The concentration of the ion is key to calculating the pH of the solution so you will need to know the moles and volume of the ion in the ion that effects the pH. Remember the Ka or Kb need for a salts effect on the pH must be calculated using Kw = Ka*Kb. The needed value is based on the member of the conjugate pair that appears as one of the products.
titration problems1
Titration problems
  • Write the reaction for the neutralization of the acid and base. Identify the ion in the salt that will effect the pH.
  • Remember the equation is balanced in a mole to mole ratio. So you need to calculate the moles or millimoles of the acid, base and ion of interest. (M = mol/L or mmol/mL) You also need the volume of the acid, base and the resulting volume that the ion is contained in. The molarity of the ion of interest is the target of this process.
  • Write the reaction of how the ion of interest will effect the pH. It will either act as a proton acceptor from water of as a proton donor to water.
titration problems2
Titration problems
  • Write the equilibrium expression. Is this a Ka expression or a Kb expression?
  • Remember the acid and base were completely neutralized and have a concentration of zero.
  • Assign the Initial concentration of the ion. Relate the Change and find the relative Equilibrium concentrations.
  • Is the given equilibrium constant the one you need or an aid to calculate the one you need.
  • Solve for the unknown and determine the pH.