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Weak Electrolyte Ionization

High schoolChemistryIdeal monoprotic acid or base charge balance including water, dilution and a common strong acid or base at 25 °C

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About this experiment

Dilute a weak electrolyte, add a common ion and compare pH, ionized fraction and estimated conductivity. The ionization equilibrium is solved from mass and charge balance rather than a square-root shortcut.

Learning goals

  • Distinguish concentration from degree of ionization.
  • Predict dilution and common-ion effects for acids and bases.
  • Relate ionic populations to an illustrative conductivity estimate.

How to use it

  1. 1Record the initial weak acid pH and ionized fraction.
  2. 2Add water and compare the fraction ionized with the falling analytical concentration.
  3. 3Reset and add HCl; observe how a common hydrogen ion suppresses acid ionization.
  4. 4Switch to a weak base and repeat using NaOH as the common-ion reagent.

The model

For a weak acid the dissociated anion fraction follows from this mass balance.
This is the acid-side charge balance with added HCl; the base-side calculation includes its positive conjugate acid and sodium.
Limiting ionic conductivities provide a dilute-solution estimate.

Assumptions

  • Water autoionization is fixed at Kw = 10 to the minus fourteen at 25 °C. Activities and ionic-strength effects are omitted.
  • The sample starts at 100 mL. Diluent and 0.1 M strong-reagent volumes contribute to the total volume; common ions are added as HCl or NaOH, not as a conjugate salt.
  • The conductivity coefficients approximate acetate, ammonium and common spectator ions. Changing pK creates an illustrative analogue rather than a new measured substance.
  • Meter colour is a virtual indicator overlay. Particle colours are representative fractions, not individual electron or molecular trajectories.

Try this in class

  1. Q1Why can ionized fraction increase while the concentration of ions decreases on dilution?
  2. Q2Why does a common strong acid reduce the weak-acid anion fraction even while total conductivity rises?

Frequently asked questions

Does this always use the square-root ionization formula?

No. The full electroneutrality equation includes water and strong electrolyte, so it remains usable when the usual small-ionization approximation fails.

Does added water change the number of weak-electrolyte formula units?

No. It changes the total volume and equilibrium distribution, while the analytical amount of weak electrolyte remains fixed.