Simple Titration Lab
In this experiment you will be determining the concentration of both a strong acid (HCl)
and a weak acid (HC2H3O2) by titration. We will use phenolphthalein as the indicator for
both titrations to visually represent the equivalence point called an end point.
To begin the lab start by rinsing your Erlenmeyer flasks, buret, and pipet with distilled
water. Then obtain a beaker of acid to be tested and pipet 10 ml of acid into each
Erlenmeyer flask. Add about 50 ml of water. Add 3 to 4 drops of phenolphthalein and
set aside. Next obtain a beaker of 0.1 M NaOH and add a small amount to the buret.
Rinse the buret with this initial amount and pour out after running a little bit through the
tip. Repeat with another small amount. Then fill the buret to some value less than the
top reading and record the initial volume. Start adding the base to the acid and slow
down to a few drops at a time when the color begins to change. Make sure you are
swirling the Erlenmeyer flask as you go so it stays evenly mixed. As the color starts to
persist try to add a drop or two at a time until you get a very pale pink that doesn’t
disappear. This point is your end point. So record the final volume. If you had to stop
and re-fill your buret make sure your record the first final volume and the second initial
volume to be added with the first amount of NaOH. Once you have reached the end point
wash out that Erlenmeyer flask and repeat for each of the other trials. Then repeat for the
weak acid.
You should record all volumes of acid, all volumes of bases (initial and final), known
molarity of NaOH.
Calculate the molarity of the acid for each trial. Determine the average molarity for each
acid.
Question
1) Why is your volume of base added just an approximation of the equivalence
point?
2) Would this technique work for a diprotic acid? Why?