Lab Activity #8
Perfecting Mass/Mole Conversions
Part A: Molar Mass and Molar Volume Relationships
For this part of the lab, the technician will have set up a display where you can look at one (1) mole of various compounds. It's interesting to see that a mole doesn't look the same for every compound.
- Molar Mass
- Using the periodic table in the back of your textbook, you can calculate the molar mass of each compound.
- Molar Volume
- Now use the densities in the lab book to determine the molar volume of each compound (in milliliters per mole). Hint: Use dimensional analysis to get these units from information that you already have.
Part B: Determining the Formula of a Hydrate
- What is a hydrate?
- A hydrate is a chemical compound (usually an ionic compound or salt) that has some water molecules loosely attached to it. Typically, when an aqueous salt solution dries, all of the water goes away, leaving only the salt behind. In a hydrate, some of the water molecules stay attached to the salt. The number of water molecules per formula unit of salt is fixed and is often a small whole number. The water molecules in a hydrate can be removed by heating the hydrate to a certain temperature. After this heating, the pure (anhydrous) salt is left behind.
- Molar mass of a hydrate.
- The molar mass of the hydrate will include the mass of the water molecules along with the salt's mass.
- Names of hydrates
- The names of hydrates are the name of the plain salt, plus a numerical prefix (like di- or tri-) and the word "hydrate." For example, calcium chloride tetrahydrate.
- Formulas of hydrates
- The formula for a hydrate has a dot between the salt formula and the formula for a water molecule, followed by a number corresponding to the number of water molecules inthe hydrate. For example, CaCl2 . 4 H2O.
In the lab, you'll be given a sample of barium chloride (something)hydrate, where the "something"
is a numerical prefix (such as mono-, di-, tri-, or tetra-). The formula is
BaCl2 . x H2O,
where "x" equals the number of water molecules attached to the barium chloride in the hydrate.
Your job will be to figure out how
many water molecules are attached to the barium chloride.
To determine the number of water molecules attached to the barium chloride in its hydrated form,
you will heat the hydrate and force the water to evaporate away, leaving anhydrous barium chloride,
which has a formula of BaCl2.
From the mass (of the hydrate) before heating and the mass (of the anhydrous salt) after heating,
you will be able to calculate the amount of water that escaped during the heating process and thus
the percent of water in the hydrate.
In order to determine the formula of the hydrate, you can calculate the value of "x" using the
ratio between the moles of water and
the moles of the anhydrous barium chloride. Both of these quantities may be obtained from the
data you have collected in this lab. Since you know the mass of water lost, you can calculate
the moles of water lost. Similarly, since you know the mass of anhydrous barium chloride, you
can calculate the moles of anhydrous barium chloride, which is equal to the moles of barium chloride
in the hydrate. The ratio of these two quantities (moles of water per moles of anhydrous barium chloride)
is the value of "x".
Alternatively, write the equation for percent water in the hydrate with an "x"
in the place of the multiplier for the number of water molecules. Then plug in the molar masses that you
know (i.e., for water and for anhydrous barium chloride) and solve for "x". Remember, the molar
mass of the hydrate is equal to the molar mass of barium chloride plus "x" times the molar mass of water.
Have fun!
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