Lab 2Determining 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.

ProcedureThis 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 ExperimentIn 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 HydrationThe 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 CopperOnce 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.

FiltrationYou 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 Questions1. 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.