To find experimentally the molar mass of magnesium
Concentrated hydrochloric acid
Long graduated glass tube sealed at one end (old burettes are very suitable for this)
Large container, eg. Pneumatic trough
Concentrated HCl is corrosive; wear eye protection and handle with care. If there is a spill of concentrated HCl it should be dilluted with a large amount of water.
1. Clean a strip of magnesium and cut off 3 cm
2. Weigh the magnesium
3. Pour concentrated HCl into the long glass tube to a depth of 7cm
4. Gently fill the tube with water, avoiding any unecessary mixing with the acid
5. Push the magnesium into the water in the tube
6. Plug the end loosely with cotton wool
7. Cover the end of the tube and invert the tube carefully into a large container. Do not let any air bubbles into the tube. The more dense acid will now sink, moxing with the water and producing dilute HCl, which will react with the Magnesium to produce hydrogen gas. Do not let any hydrogen gas escape.
8. When all the magnesium has reacted, raise the tube so that the level of water inside the tube is inkine with the level of water in the container, and read the volume of gas produced.
9. Record your observations
10. Write an equation for this reaction. Use the volume of hydrogen produced from the 3 cm Mg to calculate the molar volume mass of Mg. Assume that the temperature is at SLC and the pressure is 101.3 kPa during this experiment.
Mass of 3 cm Mg = 0.03g
Volume of Hydrogen = 5mL
Number of moles of hydrogen gas you've produced = 0.002 moles
How many moles of mg you need to get this many moles of hydrogen = 0.0032 moles
Molar mass (exp) of mg = 0.07950g
Actual molar mass of mg = 24.305g
Validity: This experiment worked out exactly as we had presumed it would. Towards the end of the process, bubbles started flowing up to the top of the test tube, followed by the strips of magnesium. This experiment was very valid.
Reliability: Some groups didn't get to finish, but overall everyone got similar results, leading us to believe that this experiment consisted of a very good reliability.