EXPERIMENT 12B: TITRATION OF AN UNKNOWN ACID
INTRODUCTION
In this experiment you will determine the molar mass of an unknown acid by
titration with the sodium hydroxide you prepared and standardized in experiment 12A.
Because you will be titrating an unknown acid again, you will be using many of
the concepts and methods learned in Titration Experiment. Besides, the pilot titration is
a new technique that you will learn.
When you titrate, it is important to choose the right sample size, one that
requires a volume of titrant that is less than the calibrated buret volume. In our case
that volume is 25 ml. Because each buret reading has an uncertainty associated with it,
it is desirable to make the smallest number of readings-two- for each titration so that you
minimize the error. If your sample required more than 25 ml, the buret would have to be
refilled, two additional readings would have to be made for a total of four readings and
additional uncertainty. In the Titration Experiment the sample size was chosen for you.
In this experiment you will determine the mass of unknown acid to use by carrying out a
pilot titration.
EXPERIMENT SUMMARY:
Your unknown solid is an acid. You will dissolve it in water, add some
phenolphthalein indicator and then titrate to the end point with your standard NaOH
solution. The unknown acid is monoprotic; it has only one acidic hydrogen per
molecule. Let’s use HX for its formula. The reaction between HX and NaOH is
HX (aq) + NaOH (aq) NaX (aq)+ H2O (l)
You will measure the volume of standard NaOH solution that is required to react
with an accurately measured mass of unknown acid. From these data and the mole
ratio of NaOH to unknown acid given by the equation above, you will calculate the molar
mass of the unknown acid.
PROCEDURE
A. PILOT TITRATION
1. Check out a buret from the stockroom. Clean the buret, then rinse and fill it
with your standard NaOH solution. (Don’t forget to swirl the NaOH before you use it.)
Take your initial reading and record it below; do not record data for this titration on your
report sheet.
2. Obtain an unknown acid sample from your instructor. Weigh to the nearest
0.01 g about 0.4 g of your unknown acid on a platform balance as follows:
a) Tare a clean dry 50 ml beaker on the platform (top-loading) balance.
b) Add about 0.4 g of your unknown acid into the beaker and record below (on
page 2) the initial mass.
c) Transfer this unknown acid sample into a clean, but not necessarily dry,
Erlenmeyer flask.
