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A common analysis of a weak acid or base is to conduct a titration with a base or acid with a known molar concentration. This will help determine the equilibrium constant, Ka, for the weak acid or base. According to the Henderson-Hasselbalch equation,.
pH = pKa + log [base/acid].
If there are equal moles of weak acid and the base, NaOH at the half-equivalency point during this lab, then the pKa is equal to the solution's pH value. The half-equivalency point can be found by first solving for the equivalency point. First, drawing two tangent lines to the flat portions of the pH curve can determine the equivalency point. Next, draw perpendicular lines to the two previous tangent lines. Find the midpoint on the perpendicular lines and then connect them by a line. The point where the line intersects the pH curve is the equivalence point. To find the half-equivalency point and ultimately the pH of the solution, simply take this value and divide it in half. The pH of the solution, which directly tells us the [H+], aids in the determination of the ionization constant. Since this is a weak-acid plus strong-base titration, the pH was not equal to 7.0 at the equivalency point Le Chatelier's Principle states that when a system that is in dynamic equilibrium is subjected to a disturbance that upsets the equilibrium, the system undergoes a change that counteracts the disturbance and, if possible, restores the equilibrium. The weak acid, HA ionizes according to the equation:.
HA H+ + A-.
The ionization will occur until equilibrium is established. As more H+ ions are created in the above equilibrium equation above, the system adjusts itself to create more H+ ions. As more H+ ions are created, more HA must be broken down into H+ and A- ions, leaving less and less HA. At some point, enough OH- ions will have been added to the solution to neutralize all of the HA present. However, if a base such as sodium hydroxide is added to an aqueous solution of HA at equilibrium, the following neutralization reaction will occur:.