Formula of a Hydrate - by Jen-Chou Wang & Tandy Grubbs

Goal:   To determine the empirical formula of a copper chloride compound.

Specific Learning Objectives:   By completing this exercise, the user will gain experience in how to make use of mass measurements before and after two chemical transformations to discern the empirical formula of a copper chloride hydrate, CuxCly•zH20.

Prerequisites:   Some familiarity with the use of scientific (exponential) notation, the concepts of empirical formula, mole, and molar mass, and how to perform mass-to-mole conversions is recommended.

Resources you will need:   A scientific calculator.


___________________________________________________________________________________________________

Background: 

Many chemical compounds exist in the form of a hydrate, meaning that a certain number of water molecules are present in the crystalline makeup of that compound.  Cobalt chloride hexahydrate provides an example.  The empirical formula of this compound, given by CoCl2•6H20, reveals that each formula unit contains one cobalt atom, two chlorine atoms, and six waters of hydration - the waters of hydration are normally indicated by adding the notation •zH20 to the end of the formula, with z representing the number of waters of hydration.  Many hydrates can be chemically converted into an anhydrous form by application of heat.  For example, heating the purplish-red cobalt chloride hexahydrate compound from above in a ceramic crucible drives off the waters of hydration and forms a blue anhydrous cobalt chloride compound; 

-  HEAT →

CoCl2•6H20 (s), hydrated form                                                   CoCl2 (s), anhydrous form.    

In this experiment, you will be determining the empirical formula of a copper chloride hydrate by closely inspecting mass measurements that have been recorded before and after a series of chemical transformations.  At the onset, we will refer to the formula of this compound as CuxCly•zH20, and therefore your goal is to determine the values of x, y, and z which represent the smallest whole numbers that can be used to indicate the proper combining ratio of copper, chlorine, and water, respectively, in this compound. 


Experimental Data Collection:

The experimental procedure consists of performing two chemical transformations and recording the mass of the sample before and after each reaction.  For the first reaction, a sample of the copper chloride hydrate (CuxCly•zH20) is placed in a ceramic crucible and is gently heated with a Bunsen burner to drive away the waters of hydration and form the the anhydrous copper chloride compound (CuxCly). The crystalline hydrate is stirred slowly with a metal spatula during this step to promote thorough heating and full chemical conversion of the sample:

1)   REACTION #1:     CuxCly•zH20 (s)     ---  HEAT  →     CuxCly (s).

For the second reaction, the CuxCly is dissolved in distilled water in a small beaker, forming a blue solution that is indicative of dissolved Cu(II) ions. A large coil of aluminum metal is then inserted into this solution. The copper ion in solution is chemically reduced by the aluminum metal - a reaction that involves a transfer of electrons from aluminum atoms to the dissolved Cu(II) ions, which cause the copper to precipitate out in the bottom of the beaker and the aluminum to dissolve away into solution as Al(III) ions.  The copper is subsequently collected, washed, filtered, dryed, and then weighed using a balance.

2)   REACTION #2:    CuxCly (aq)  +  Al (s)    ---→    Cu (s)  +  aluminum chloride solution.

Again, mass measurements are made before and after each reaction, meaning that there are a total of three masses at the conclusion of the experiment: A) the mass of the original hydrate CuxCly•zH20, B) the mass of the intermediate anhydrous form, CuxCly, and C) the mass of the final copper product.  Subtraction of the final copper mass from the intermeditate anhydrous mass gives the mass of chlorine in the original compound.  Subtracting the intermediate anhydrous mass measurement from the original hydrate mass measurement gives the mass of water in the original compound.  Thus, this series of chemical reactions has allowed us to break down this compound and determine the mass of its individual components.

 


The three different forms of copper investigated in this experiment - the hydrated chloride, and anhydrous chloride, and elemental copper. Green hydrated copper chloride, being converted to the anhydrous form by heating in a crucible (reaction #1).


Before-after comparison of reaction #2: anhydrous copper chloride solution prior to reduction (left), precipitated copper metal (middle), and the aluminum metal coil that was inserted in the solution to achieve this transformation. Brown anhydrous copper chloride, obtained by carrying out reaction #1.

Empirical formulas of compounds DO NOT tell you the relative masses of the components that combine in a compound.  Instead, they tell you the relative number of particles of each component that combine.  The unknown values of x, y, and z in the empirical formula above can also be interpreted as representing the relative number of moles of each component that combine in this compound.  Therefore, in order to determine the empirical formula of the original compound from the corresponding masses of the components, you will first need to convert the mass readings into the moles of the corresponding components.  Converting from mass to moles involves dividing the mass (in grams) of a substance by its molar mass:

moles of substance = (mass of substance) / (molar mass of substance).


As an example, consider a compound given by the formula CxHy.  A sample of this compound is carried through some chemical analysis and found to contain 3.0 grams of carbon and 1.0 gram of hydrogen.  The empirical formula IS NOT equal to C3H1, but is instead found by ...

  1. Converting the masses of carbon and hydrogen into the corresponding moles of carbon and hydrogen (by dividing each mass by the molar mass of that element) and
  2. Dividing the moles of each component by the smallest value to discern the proper combining ratio of each component.
A periodic table can be used to obtain the needed molar masses.  Converting to moles of carbon and hydrogen, one obtains

moles of carbon = (3.0 g) / (12.011 g/mol) = 0.25 mol and
moles of hydrogen = (1.0 g) / (1.008 g/mol) = 0.99 mol.

Dividing each of these results by the smallest of the two (0.25 mol) gives the proper combining ratio;

amount of carbon = 0.25 / 0.25 = 1  and  
amount of hydrogen = 0.99 / 0.25 = 3.96 ≈ 4.

So the empirical formula of this particular carbon-hydrogen compound would be CH4.

Experimental Data:

Mass readings associated with three trials of the experiment are included below (recorded to 0.001 grams):

Compound Trial 1 Trial 2 Trial 3
CuxCly•zH20 (s) 1.009 g 1.020 g 1.004 g
CuxCly (s) 0.796 g 0.795 g 0.788 g
Cu (s) 0.372 g 0.380 g 0.376 g
  

Analysis:

  1. For each trial, use the provided mass reading to determine the mass of copper, chlorine, and water in the original compound.
  2. For each trial, convert the masses of copper, chlorine, and water into their corresponding mole amounts, and use these quantities to determine an empirical formula for the copper chloride hydrate.
  3. Comment on the reproducability of your results (i.e. are you able to state with higher confidence your result by performing some type of average for the three trials?).
  4. Consult an appropriate literature source and determine the oxidation state(s) that are normally formed by copper ions and chloride ions.  Based upon your results, does the empirical formula that you determined from the above analysis seem plausible?  Explain.
___________________________________________________________________________________________________

Jen-Chou Wang

Tell us about yourself