EXPERIMENT 3: FINDING THE MASS PERCENT OF WATER IN A COMPOUND
Introduction (Silberberg, sections 2.8, and 3.1)
The Law of Constant Composition states that all samples of a pure compound must contain
the same elements in identical proportions, by mass. Sometimes, elements are combined into
units within the larger domain of a compound, so we make the following slight change in the
above statement: all samples of a pure compound must contain the same components in identical
proportions, by mass. Thus, the mass composition of the compound copper (II) nitrate
tetrahydrate, Cu(NO3)2•4H2O, can be reported as 24.48% Cu, 10.79% N, 61.63% O, and 3.10%
H, or 24.48% Cu2+, 47.77% NO and 27.75% H2O, or 72.25% Cu(NO3)2 and 27.75% H2O.
You might think that the composition of a compound could be determined experimentally by
finding out the mass of each of its elements that react to form an amount of the substance, or the
mass of each of its components that react to form some amount of the substance. In reality, such
a process is always very tedious, inaccurate, and often involves handling materials that are
inconvenient to manage in the laboratory. It is much easier, in general, to start with a compound
and decompose it into other substances that are well-behaved, and then relate the amounts of
those substances back to the original compound and its make-up. We will use this second
approach to find out the percent of water in a particular compound.
The underlying idea in this experiment is simple: an ionic substance that contains water
within its crystalline structure will be heated, and the water driven off, so that the sample loses
weight. The ions themselves cannot be boiled away (at least, not at the temperatures employed
here); thus, the weight loss is due only to the loss of water. The lost weight is directly
proportional to the amount of water in the sample, and may be used to calculate the percent of
water in the compound. By knowing the percent of water in the compound, it is often possible to
distinguish one compound from another. This is accomplished by first determining the
theoretical percent water for each of the compounds and comparing the determine values to
General Safety Precautions
YOU MUST REFER TO THE MSDS’s OF EACH POSSIBLE UNKNOWN AND INCLUDE
THE PERTINENT HAZARDS AND COLLECTION INFO IN YOUR NOTEBOOK
Substance Precautions Disposal
Inorganic salt hydrates, Do not ingest. Contact Left-over compound,
your "unknown" may cause dermatitis. and decomposition
May be toxic. products: waste
containers in hood.
General Precautions for this Experiment
You will be working with hot objects in this experiment, and should make sure that you
handle things with tongs, as appropriate to avoid injuries.
When you weigh crucibles, you must weigh them in clean, dry beakers, and not directly on
the pan of the balance!
Two (2) Coors crucibles
Two (2) Fisher burners, with tubing to fit
Two (2) ring stands
Mass %H2O page 1 of 2
Clean both crucibles with soap and water, and rinse both with de-ionized water before wiping
them with clean paper towels. Make sure that you can differentiate between these two
crucibles. Place each in a clay triangle on a ring mounted on a ring stand, and heat each gently
with a Fisher burner before heating both of them red-hot for five full minutes. Remove them
with tongs, and place both in a desicooler, making sure not to contaminate either one with the
contents of the desicooler, or dirt on the tongs. Cool both to room temperature, and weigh both
accurately by difference, in a clean, dry beaker whose weight is also accurately known.
From the balance room, obtain a sample for analysis. Record its number (or letter) in your
notebook. To each crucible, add approximately 2.0 grams of your sample, accurately weighed
(be sure to record the actual weight in your notebook).
Place each loaded crucible in a clay triangle as before, and heat each gently with the Fisher
burner for a few minutes--rapid heating may result in the crystals shattering, and mechanical loss
of part of your samples. Then heat them red-hot for ten full minutes. Your sample may liquefy
when extremely hot, or it may liquefy and then solidify at higher temperatures--such behavior
will not affect the outcome of your experimentation. Use tongs to place each in a desicooler,
again exercising caution to avoid any contamination.
After both your crucibles have cooled to room temperature, weigh each, and then return them
to the clay triangles for an additional five minutes' heating at red heat. Again remove them,
place them in a desicooler until they are at room temperature, and weigh each accurately--each
one should weigh within 0.0020 grams of the last weighing, but if not, then repeat the heat-cool-
weigh cycle until they do. Calculate your results, based on the LIGHTEST weights of the
crucibles with their dry contents, and record them in the spreadsheet on the designated computer
in the balance room. Be sure to record your values under the appropriate unknown identification
and to calculate the average of the values for your particular unknown with standard deviation.
Calculate the theoretical % water in the following possible substances: Cu(NO3)2∙4H2O,
CuSO4∙5H2O, CoCl2∙ 6H2O, MgSO4∙7H2O. Compare your experimental values to these values to
determine the identity of your unknown. Calculate absolute % error based on which unknown
you believe you have.
your sample number, your separate values for the water content in your sample, the average
of these values (if more than one determination was undertaken), the class average value for
the water content, and the standard deviation for these values;
discussion of any errors you committed during all the parts of the experiment, and how they
affected your results.
which of the possible unknowns you believe your sample is and why. Also include the
absolute %error for your determinations, assuming this unknown.
why you believe your values and the class average differ from the calculated theoretical
% water for that compound. These should include observations that you made that would
cause error and critical steps and measurements that could have caused the variance.
Mass %H2O page 2 of 2