Determination of a Formula Page 1 of 8
CH 221 – Experiment DETERMINATION OF A FORMULA
Purpose: The purpose of this experiment is to determine empirically the formula for the
compound formed when magnesium metal is allowed to react with chlorine in the form
of hydrochloric acid. Results from every group should be checked and compared with
those from at least one other group to develop your skills at communicating the results
of experiments with each other. This experiment will also allow us to use separations,
measurement, and precision evaluation skills developed in previous labs. Today, also,
we will be starting to validate the results of others.
Introduction: In determining an empirical formula, a chemist might start with a sample of
the pure compound, decompose it carefully into its component elements, and obtain the
mass of each element present. In some happy circumstances, the compound can be
made reliably, quantitatively and uniquely from its component parts, and some (or all) of
the component elements can be weighed accurately before the reaction occurs. The
experiment today is one such case. We will take a sample of magnesium, carefully
purify it, weigh it, and then react it. The compound formed will contain all of the
magnesium we started with since magnesium will be the limiting reagent. Any increase
in mass should be due to the other element, in this case that element is chlorine. The
reaction will take place in solution, so there will be much water that must be removed
before the compound can be considered pure. All the skills learned in previous labs
(removing water from a salt solution, and preparing glassware for the analysis) must be
used to guarantee success. Be sure that you take precautions to avoid errors, not just
use excuses that you did not do what you knew you should have avoided doing to
explain why your results might be unacceptable.
Magnesium is a metal that comes in many forms, but today it will be in the form of a flat
narrow ribbon of metal. Magnesium easily forms an oxide coating. The sample that
you get today will have this coating on it, which will make it look dull, as well as weigh
more. The magnesium oxide coating must be removed by polishing it with steel wool
until it looks shiny. Excess exuberance in polishing will only remove pure magnesium,
reduce the mass, and contribute to a smaller sample of product.
Once the magnesium is purified, it is weighed, and placed into a clean dry beaker, that
has also been weighed. The magnesium is then allowed to react with an excess of
hydrochloric acid in solution. Once this is done, the excess hydrochloric acid is
converted to hydrogen chloride gas and evaporates. This is gas is not pleasant or
necessary for a healthy individual. Although we will be generating small amounts of it,
we will further minimize the potential exposure to this by performing this reaction and
separation step in the hood. The water is removed by evaporation, just as it was last
week, so all of the precautions for last week’s lab still hold.
Recall the advice you recorded for yourselves last week about recovering a salt from a
water solution. Most of you indicated that heating to salt solution more slowly at the end
of the evaporation was a good idea to minimize product loss due to “popping.” It is also
a good idea to stop adding heat when solids start to form, let it cool down a bit and then
