Lab 2
Determining Chemical Formula
Pre-lab
 Pre-labs will be due at the beginning of your first period of AP chem
on day 1.
 You pre-lab should contain…
 Purpose:
 Explain the main goal of the lab and briefly describe how we will attain it.
 Materials:
 List ALL of the chemicals and materials needed to perform this experiment.
 Safety:
 This section should highlight ALL of the safety considerations for the lab. I
will post MSDS sheets on my teacher page for any chemicals we will be
using.
 Procedure
 This section should list ALL of the steps you are taking in
this lab in detail.
 This does not have to be in paragraph form because you
will be creating it yourself.
 Your procedure section should be detailed enough that
someone else could do the same experiment you did and
get the same results.
 Data Table
 It is on you to create your own data table.
 It should include all important measurements.
 Anything you feel is important enough to write down
should have a spot on the data table.
 Don’t forget to include spots for calculated observations
such as the mass of the hydrated sample. (found by
subtraction)
Law of Definite Proportions
 This law states that the mole ratios of elements in a
compound are always the same.
 The Law of Definite Proportions states that the same
elements can combine in different ratios to form different
compounds.
 Example:
 FeS and Fe2S3
 If we were to determine the mass of iron and the mass of
sulfur present in a sample of Iron Sulfide we could
determine the true chemical formula.
Our Experiment
 In this experiment you will determine the chemical
formula for a hydrated compound containing copper,
chlorine, and water molecules that are locked in the
crystal structure of the solid compound.
 We will use the general formula:
 CuxCly * z H2O
Water of Hydration
 The first thing we will do is determine the water of
hydration.
 This will be done just like our first lab.
 You will use the same equipment, set up, and basic
procedure as lab number one.
 Remember to record the mass of the empty crucible
and lid so that we can determine the mass of our
sample accurately (through subtraction).
Determination of Copper
 Once we have dehydrated our sample we can
determine the amount of copper.
 Once we rehydrate our sample (this will be done by
washing it out of the crucible with water into a beaker)
we will end up with a blue colored solution.
 This solution contains copper ions, this is what makes it
blue.
 These copper ions will react with aluminum to form
solid copper metal.
 You will obtain about 20 cm of aluminum wire.
 It is important that we make sure we have enough aluminum
to react with all of the copper in solution so that all of the
copper ions are converted into solid elemental copper.
 This reaction should be allowed to run for at least 30 mins or
until the solution loses its blue color
 This will be our signal that all of the copper ions have been
converted to elemental copper.
 You will then add 5 drops of 6 M HCl to dissolve any solid
impurities that may have formed.
 Make sure you rinse all of the copper off of the aluminum
wire using distilled water.
 If any of the copper is really stuck you can use one or two
drops of the 6 M HCl to remove it.
Filtration
 You will then filter the copper from the liquid solution.
 We will use the same equipment, and procedure that
we practiced in advanced lab skills.
 WE NEED TO MAKE SURE THAT WE RECORD THE
MASS OF THE FILTER PAPER BEFORE WE START
OUR FILTRATION!
 Once we filter and rinse our sample of copper we can
let it dry and record its mass.
Mass of Chlorine
 How will we determine the mass of chlorine in our
sample?
 Simple subtraction
 (Mass H2O) + (Mass Cu) + (Mass Cl) = Total Mass
Post Lab Questions
1. How many moles of water were in your sample?
2. How many moles of copper were in your sample?
3. How many moles of chlorine were in your sample?
4. Show your calculation of the proper chemical formula
for the compound that we tested.