Solving an acid base problem

(by looking at it and thinking about what happens)

This problem set was developed by S.E. Van Bramer for Chemistry 145 at Widener University.

Identify Each Species.

First identify each species and decide what it will do in water
  1. Is it a strong acid? If yes, it will completely dissociate in water. Since it is the ions that do the chemistry, write them as ions. example:

    HCl + H2O -> H3O1+ + Cl1-

    Since it is a strong acid it dissociates completely, you will not have any HCl. Cl1- is not going to do anything in an acid base reaction. It is a spectator ion so leave it out. What you need is to figure out the [H3O1+] (ie: the concentration of hydronium ions). Remember concentration is moles over liters.

  2. Is it a strong base? If yes it will completely dissociate in water. Since it is the ions that do the chemistry, write them as ions. example:

    NaOH + H2O -> Na1+ + OH1- + H2O

    Since it is a strong base it dissociates completely. You will not have any NaOH. Na1+ is not going to do anything in an acid base reaction. It is a spectator ion so leave it out. What you need to do is figure out the [OH1-] (ie: the concentration of hydroxide ions). Remember concentration is moles over liters.

  3. Is it a weak acid? If yes it will partially dissociate in water. Now you need to pay attention to the equilibrium reactions. First write out the balanced equilibrium reaction. example:

    HF + H2O <-> H3O1+ + F1-

    Then write out the equilibrium expression:

    You will need to use this expression to determine what concentration each species is at when the system is at equilibrium. Exactly how to solve this depends upon what other species are present. More on that coming up.

  4. Is it a salt? What will it do in solution. FIRST, the salt will dissociate into ions. Just like NaCl in water dissociates and forms Na1+ and Cl1-. Any other salt will do the same thing. You should be able to identify a salt by looking at he periodic table and thinking about trends in electronegativity. First let the salt dissociate. example:

    NaF(aq) -> Na1+ + F1-

    You need to do this so that you can decide what will happen in solution. Almost everyone in the class missed one or more problem because they failed to recognize a salt, and then see what it will do in solution. After you write the salt out as ions, look at the ions to see if you recognize any of them as a weak acid or a weak base. Or the conjugate acid or base. Anything with a Ka or Kb. After you recognize it as an acid or base, write the appropriate reaction.

    This step can be a bit tricky. For the example here F1- is the conjugate base of a weak acid. That means that it would like to gain a proton (H1+) from something. The reaction that you write depends upon what species are present. The idea here is to think about what the strongest acid is. If only the salt is present, the strongest acid is water.

    F1- + H2O <-> HF + OH1-

    But, If any acid is present you can write a different reaction.

    F1- + H3O1+ <-> HF + H2O

    Notice that this is like the acid dissociation of HF shown above. Since HF is a weak acid, F1- is also a weak base. That means you need to use an equilibrium expression to solve for the concentration of each species. We will deal with the equilibrium expressions below.

Write out a list of all species present.

After you identify all the species present and write out appropriate reactions, write out a list of all the species present, ignoring equilibrium reactions and species at first.
  1. If you have a strong acid, write down [H3O1+]
  2. If you have a strong base, write down [OH1-]
  3. If you have a weak acid, write down [HA]
  4. If you have a conjugate base (from a salt), write down [A1-]

Deal with the strong acid and base.

THESE WILL REACT TO COMPLETION WITH THE AVAILABLE SPECIES.
  1. A strong acid will react with a strong base.
    1. Determine which is the limiting reagent.
    2. Find the final concentration of the excess reagent.
    3. The final concentration of the excess reagent will determine the pH. DONE.

  2. A strong acid will react with a conjugate base.
    1. The reaction will go to completion.
    2. Determine which is the limiting reagent.
    3. If conjugate base is limiting, there is excess strong acid. This determines the pH. DONE.
    4. If the strong acid is limiting, there will be excess conjugate base and the pH is determined for the buffer (produced from the conjugate base and the weak acid)

  3. A strong base will react with a weak acid.
    1. The reaction will go to completion
    2. Determine the limiting reagent.
    3. If the weak acid is limiting, there is excess strong base. This determines the pH. DONE
    4. If the strong base is limiting, there will be excess weak acid and the pH is determined for the buffer (produced from the conjugate base and the weak acid).

Buffer.

If you could not determine the pH from above (By an excess of strong acid or strong base) then you have a buffer solution and need to determine the pH from that.
  1. If you only have a weak acid.
    1. Determine the concentration of the acid (assuming that there is no dissociation).
    2. Look up or determine Ka.
    3. Write an expression for Ka from the reaction: HA + H2O <-> A1- + H3O1+

    4. Since [HA] and Ka are known, Let [A1-] and [H3O1+] be X and solve as:

      1. If X is much smaller than [HA], ignore X in [HA - X] and solve.
      2. If X is similar to [HA], solve with the quadratic equation.
      3. Determine pH from [H3O1+]. DONE.

  2. If you have a weak acid AND the conjugate base. Solve for the buffer.
    1. Determine the concentration of the weak acid and the conjugate base (Ignoring any equilibrium effects at first )

    2. Look up or determine Ka.

    3. Write an expression for Ka from the reaction: HA + H2O <-> A1- + H3O1+

    4. Since [HA], [A1-], and Ka are know: Let [H3O1+] be X and Solve
      1. If X is much smaller than [HA] or [A1-], ignore X in [HA - X].
      2. If X is similar to [HA] or [A1-], solve with quadratic.
      3. Determine pH from [H3O1+]

  3. If you only have the conjugate base. Solve for the pH of the base using Kb and the hydrolysis equation.
    1. Determine the concentration of the conjugate base.
    2. Determine Kb (using Ka if necessary)
    3. Write an expression for Kb from the reaction: A1- + H2O <-< HA + OH1-

    4. [A1-], and Kb are known: Let [HA] and [OH1-] be X and solve as:

    5. Solve for X
    6. Determine pOH from [OH1-], then determine pH

Please send comments or suggestions to svanbram@science.widener.edu

Scott Van Bramer
Department of Chemistry
Widener University
Chester, PA 19013

© copyright 1996, S.E. Van Bramer
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Last Updated: Monday, May 27, 1996