Atomic Mass of a Metal

 

            The purpose of this experiment is to test the accuracy of a method used to determine the atomic mass of metals.  To test this method, you will be asked to perform many calculations in addition to the procedure itself.  Keep in mind throughout the day that you will be asked to critique all aspects of the procedure.  Look for any possible procedural error that might occur. 

            This method is based upon reacting a known mass of a metal with concentrated (12 molar) hydrochloric acid.  The metals we will test are magnesium and zinc.  They will react with the hydrochloric acid according to the following chemical equations: 

 

            Zn (s)     +      2 HCl (aq)     à       ZnCl₂ (aq)         H₂ (g)     

 

            Mg (s)     +      2 HCl (aq)     à       MgCl₂ (aq)         H₂ (g) 

 

These reactions will be run in a eudiometer tube (your lab instructor will show you a eudiometer tube and the set-up necessary to perform the experiment).  A eudiometer tube is designed to catch the gaseous products from a chemical reaction, and measure the volume of that gas.  Ask your instructor if you have any questions about the procedure. 

            Your will need to do some calculations and make some decisions before you begin the procedure. 

1.       What values should be used to determine the atomic mass of the metal?

2.       How can these values be determined experimentally? 

3.       How much metal and hydrochloric acid should be reacted together? 

4.       Which reactant should be limiting? 

5.       What data need to be collected?

Answer these questions in the procedure part of your lab report.  Be sure to label your answers – 1, 2, 3, etc. 

 

Be sure to collect all necessary data.  When in doubt, record it.  After the procedure is completed, you will need to consider some other questions.  Answer these questions in the conclusion part of your report.  Again, label your answers. 

1.       Are the moles of limiting reactant an important value for determining the usefulness of this procedure?  Explain. 

2.       Does the volume of gas collected accurately relate to the volume of hydrogen produced?   Explain. 

3.       Does the temperature of the “collection water” accurately relate to the temperature of the hydrogen gas?  Explain. 

4.       What happens if hydrogen dissolves in water?

5.       What happens if bubbles are present in the eudiometer tube before the hydrogen is collected?  Is this a problem?

6.       Can we correct for the “siphon effect” and for water vapor?  Explain. 

 

Use our usual report format – purpose, procedure, data/results, conclusion.  Show samples of your calculations.  Take time to format your data table neatly.  Be sure your conclusion contains a brief critique of the procedure.  Include the percent error for your determination and the percent error for the class average.  Do you recommend this procedure?  Why/why not?