The Experiment Results on Determination of Morality of an Unknown Solution in an Acid-base Titration

Titrations allow the calculation of the concentration of a solute in an unknown solution, based on a standard solution, where the concentration is known. Acids and bases combine in solution to form water and a salt, essentially neutralizing each other. For example, sodium hydroxide, which is a base, neutralizes acetic acid, forming the product of sodium acetate and water.1 The chemical reaction reaches the equivalence point when the two solutions are the same stoichiometrically. For acid-base titrations indicators can change color from a change in pH, and this can help determine where the equivalence point is.

A buret is cleaned with distilled water and rinsed with about 5 mL of the NaOH solution, then filled to the 0.00 mL mark with NaOH. The level of the NaOH was recorded. A volumetric pipet was used to fill a 125 mL Erlenmeyer flask with 5.00 mL vinegar, (also known as acetic acid,) and 3 drops of phenolphthalein were also added to the flask. The flask was placed under the buret and NaOH was slowly added to the flask, while swirling it. Once a slight color change from clear to pink began, no more NaOH was added to the flask and the volume of the buret was recorded. The buret was filled again with NaOH, and this entire process was repeated 3 more times for a total of 4 trials.

The volumes of liquid in this experiment were critical because 1 drop of NaOH had the potential to change the color of the phenolphthalein in solution from clear to purple. A modified version of the dilution rule was used to calculate the molarity of acetic acid. The concentration of acetic acid reported by the producer was calculated by converting g/mL to mol/L or M, in order to compare it to the calculated value of concentration of acetic acid. The two values were very close, and the percent error was well below 5%, so the experimental procedure doesn’t need to be changed.

This experiment demonstrated how the molarity of an unknown solution in an acid-base titration can be determined by using the known volumes of solution and a standard solution. The rapid change of color in the phenolphthalein indicated how the equivalence point can be very hard to achieve once the pH approaches that of the equivalence point. A single drop of NaOH when the solution is close to the equivalence point can change the pH drastically. The percent error for the calculated molarity of acetic acid was 0.120%, so the experiment is valid.