The Law of Definite Composition

Note: This experiment has a significant number of hazards associated with use of the burner and crucible. The goggles must be worn by every student for the entire duration. Only after the last flame is extinguished may any student remove goggles.

Day One

1. Mass the clean dry crucible and lid.
2. Set up a Tirell Burner with a ring-stand and iron ring, or tripod, and pipe-stem triangle. Make sure that the sizes of these elements are compatible with each other. Place the lid on the crucible in a “cocked” fashion that permits air to enter and leave the inside of the crucible freely, but heats the lid as well. Have crucible tongs and a square of iron gauze at the ready.
3. Adjust the flame on the burner until a “cone within a cone” is visible. Place the tip of the inner cone at the bottom of the empty, clean, dry crucible. Heat slowly at first (by moving the burner to and then away from the crucible), then more strongly (by leaving it directly on the crucible until the bottom glows orange for about five minutes). From this point forward, handle the crucible and lid with tongs only. Place the crucible in a small, clean, dry beaker with a watch glass over the top. Place in your lab drawer for tomorrow.

Day Two

1. Collect a sample of Mg ribbon from me. Clean it with a file or sandpaper (so that it is uniformly shiny). Handling it with brown paper only from now on, form the ribbon into a loose coil that will fit completely within your crucible.
2. Mass the empty crucible and lid. Place the coiled magnesium in the crucible (still with the lid) and mass again. Record both values.
3. As yesterday, heat slowly at first then more strongly. After a time the magnesium should ignite (you will see an intense white hot glow from the uncovered portion of the crucible for a few seconds). Once the magnesium burns out you may push the lid over the crucible and continue strong heating for five more minutes.
4. Allow to cool for five minutes. Add ten drops of distilled water and once again heat strongly for five minutes, this time without the lid.
5. Allow to cool for as long as possible (check the time). At the last possible instant mass the crucible, lid and contents. Record.

Report

1. Purpose: To verify that compounds form in small, whole number ratios.
2. Hypothesis: Make an educated guess: you can read, use the index, do some research. Figure out why the ratio of Mg:O ought to be one of the following and not the others, and express that guess and reason as a hypothesis. Do so before coming to class on Wednesday. Possible ratios are: 1:1, 1:2, 1:3, 2:1, 2:3, 3:1 and 3:2. Make sure you justify your choice with more than the fact of the formula.
3. Procedure: as before, incorporate these instructions by reference. Spend your time analyzing and thinking about conclusions.
4. Data: Complete the table below:

1. Analysis: Show every, excruciating detail of your calculation.
1. Calculate the moles of magnesium present at the start: Moles = mass / molar mass. The molar mass of magnesium is 24.31 g/mol.
2. Calculate the mass of oxygen incorporated into the ash (assume all mass gained is from oxygen). The molar mass of oxygen is 15.9994 g/mole.
3. Calculate the ratio of moles of oxygen to moles of magnesium in the finished compound.
4. Write a formula in the form of a small whole number ratio (you’ll need to round a bit, probably): Mg xO y.
5. Complete the table.
2. Error analysis:
1. Take the accepted value of Mg:O to be 1:1 (or just 1).
2. Calculate the ratio of Mg:O based on your data (using the values for the number of moles obtained in 13a and 13b.
3. Determine absolute error and relative error.
3. Conclusion:
1. Was your work accurate (+ or – 5% error for the ratio is pretty accurate given our limitations).
2. Was your hypothesis correct?
3. Why or why not accurate?
4. What could be done better? Differently?