Lab #6  Mole-to-Mole Relationships in a Chemical Reaction 
General Discussion
In this experiment you will carry out a reaction between metallic copper atoms and silver ions in solution to produce copper ions in solution and metallic silver atoms.  The objective will be to determine the mole ratio between silver and copper in the reaction.
 
The equation, shown without coefficients, is:
Cu(s)  +  Ag+(aq)  ------>  Cu+2(aq)  +  Ag(s)
The symbols in parentheses, tell the physical state or condition of the atom or ion involved.  Once the mole ratio is determined, it may be converted to a whole number ratio, and the numbers placed in front of the symbols for copper and silver.  The result should be a balanced equation with the atoms of  product equal in number to the atoms of reactant.  Stoichiometric calculations are based on the coefficients in a correctly balanced equation.
 
Objectives:
1.  To experimentally determine the mole ratio between the participants in a chemical reaction.
2.  To experimentally determine the coefficients in an equation for a reaction.
 
Materials:  Copper wire, balance, 250-mL beaker, silver nitrate solution, retort stand, wire quaze
 
Procedure
1.  Obtain a piece of  heavy copper wire about 20 cm long.  Form the lower part into an elongated coil, and bend it to form a hook at the opposite end.  Carefully weigh the copper to the nearest 0.01 g and record the mass in the data table.
2.  Carefully weigh a clean, dry, 250 mL beaker to the nearest 0.01 g.
3.  Add about 150 mL of 0.2 M silver nitrate solution (0.2 moles of silver nitrate in 1 Litre of solution) into the weighed beaker.
     Caution: Silver nitrate solution stains.  If you get any on you or spill any on the desk wash it off immediately.)  Check with the teacher for advice on removing stains.
4.  Suspend the coil of copper in the silver nitrate solution by hanging the hook over a glass stirring rod or over the side of the beaker.  Do not let the coil touch the bottom of the beaker.
5.  Place the beaker on the counter top in a quiet place until the next day.
6.  Carefully shake the silver from the copper coil into the beaker.  Using a wash bottle rinse the coil and then allow it to dry.  Weigh and record its mass.  If red particles of copper appear in the solution, add 3-5 mL of the 0.2 M silver nitrate solution.  Decant the solution.
7.   Add 10 to 15 ml of distilled water to the silver in the beaker and then carefully decant (pour the water off).  Repeat the washing process at least three times.  Do not be concerned if a few silver crystals are lost in the process, as this loss is usually negligible.
8.   Dry the silver in the beaker by placing it on a wire quaze and warming until the water is evaporated off.  Heat and reheat until consecutive weighings agree within 2%.
 
Data Table
1.    mass of copper coil before reaction                           ______ g
2.    mass of copper coil after reaction                              ______ g
3.    mass of copper used in the reaction                           ______ g
4.    mass of beaker and dry silver                                    ______ g
5.    mass of empty beaker                                               ______ g
6.    mass of silver produced in reaction                            ______ g
7.    moles of solid copper used in reaction                       ______ mol
8.    moles of solid silver produced in reaction                  ______ mol
 
Followup Discussion
     The numbers obtained for items 7 and 8 in the table represent the mole ratio of Cu and Ag.  These fractioanl numbers may be used as coefficients in front of the symbols in the equation.  However, it is generally desirable to use whole number coefficients whenever possible.  A close approximation of whole number ratios may be obtained by dividing both experimentally determined coefficients by the smallest of the two numbers; that is, the moles of copper.  When a chemical equation is correctly balanced, the number of atoms on the left and right sides of the equation are balanced, as are the units of electric charge.
 
Follow-up Questions
1.  Write the equation for the reaction between copper and silver ion.  Include your experimentally determined mole ratios as fractional coefficients.  Convert the fractional coefficients to a whole number ratio and rewrite the equation using the whole number ratio.  How do these ratios compare with the correct whole number ratios furnished by the teacher?
 
 
 
 
2.   List some of the sources of experimental error in this experiment.
 
 
 
 
 
3.   If the silver in the beaker contained water during your last weighing, how would this affect your results?
 
 
 
 
 
4.   Assume that magnesium metal would act atom-for-atom exactly the same as copper in this experiment.  How many grams of magnesium would,  have been used in the reaction if one gram of silver were produced?  The atomic mass of magnesium is 24.31 g/mol.
 
 
 
 
5.  Account for the blue colour produced in the solution.