Acid Base Titrations

Topics: Acid, Acid dissociation constant, PH indicator Pages: 1 (310 words) Published: September 28, 2008
The acid-base titration curves help to find the pKa, Ka, and pH at equivalent point. At the beginning pH for HCl is 1.90 which is lower than the 3.28 for acetic acid; thus, strong acid (HCl) means lower pH and weak acid (acetic acid) means higher pH. Then at the equivalent point for the titration HCl-NaOH the pH is 7, which mans that is neutral, in other words there are enough NaOH mmol to neutralize the HCl mmol present; also, the solution contains only water and NaCl the salt derived from the titration made. On the other hand, at the equivalent point for the titration acetic acid-NaOH the pH is 8.67; consequently is more basic. This is something that it was expected because for a weak monoprotic acid (acetic acid)-strong base (NaOH) titration the pH at equivalence point is always grater than 7 because the anion of the weak acid is a base. Next beyond the equivalent point, the curves for the acetic acid- NaOH and HCl-NaOH titrations are identical because the pH in both cases is determined by the concentration of OH from the excess NaOH., getting all this data the pKa is determined by estimating the pH at the volume point which is 16 divided by two which is 8 (the halfway to the equivalent point), so when the volume is 8, pKa is 4.76, taking the antilog of 4.76 the answer gives a value of 1.74 * 10 ^-5 that is very accurate to the theoretical one. The indicator plays an important role for determining the equivalence point based on the color change. Phenolphthalein is an excellent indicator for the acetic acid-NaOH titration because the pH at equivalent point (8.67) falls within the pH range (8.2-9.8) in which phenolphthalein changes color,.
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