Indirest determination of the mass of Magnesium Ribbon
March 12, 2007
Indirect determination of the Mass of a Sample of Mg Metal
Procedure:
1) Fill a 50ml eudiometer tube with water. Cover the opening with a finger or thumb and invert the tube in a large beaker which is about 80% full of water. If necessary practice a few times until you can invert the tube without getting any bubbles in the tube. Empty the tube.
2) Obtain a sample of magnesium from your instructor. Tie about 20cm of thread around the a loosely folded piece of magnesium ribbon.
3) Pour 8 to 10 ml of 6M HCl into the empty gas burette. Carefully fill the burette with water from a small beaker.
4) Put the magnesium coil into the water in the eudiometer tube, leaving the thread hanging on the outside. Then invert the tube as practiced above, making sure the magnesium strip remains inside the tube. Avoid air bubbles.
5) When all the magnesium has reacted, note and record the following:
a. Volume of hydrogen produced.
b. Temperature of the water
c. Atmospheric pressure
d. The difference in height(in mm) of the water levels inside and outside of the eudiometer tube, using a ruler.
6) When the reaction is complete, the atmospheric pressure on the outside will just equal the pressure exerted by the column of water in the eudiometer tube plus the partial pressures of the hydrogen and the water vapor.
Patmoshere = PHydrogen + Pwater + ht. of water column (mm)/13.6
Phydrogen = Patm – Pwater – ht. of water column (mm)/13.6
The pressure exerted by the column of water is equal to the height of a column of mercury containing the same weight as the column of water. Since the density of mercury is 13.6 g/ml and the density of water is 1.0 g/ml, the height of the column of water must be divided by the ratio of these densities, which is 13.6.
Pwater = vapor pressure of water at the temperature at which the gas was collected.
Find the mass of your sample by using PV=nRT.
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Conclusion: What did you learn?
How were you able to determine the mass of the Mg ribbon when it was too light to be accurately measured on our balances?
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