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erobinson · Uploaded by **erobinson** **(60)** on Apr 29, 2004

The Molar Volume of Gases

Introduction:
Using FeCl3 to decompose H2O2 and then HCl to decompose Mg we will determine the volume of gas using the state of gas law. The purpose is to learn how to determine molar volume of gas

Data:

Discussion:
The purpose of this lab was to learn to determine molar volume in an experimental environment utilizing two different methods. The first method was by determining the weight of the flask containing H2O2 before and after the catalyst FeCl3 was introduced, thereby determining the mass of oxygen. We found the mass to be 0.256g, which is 0.008 mol. From there we were able to determine the volume of oxygen by looking up the water vapor pressure at the recorded temperature, and subtracting that from the current barometric pressure to give the pressure in torr for oxygen (739.3 torr), then utilized the state of gas law V2 = V1 (P1 / P2) * (T2 / T1). We found V2 to be 0.192L. Simply dividing the volume of O2 by moles of O2 (hence L/mol = Molar Volume), we found the molar volume of oxygen to be 24L/mol. This is only off by approximately 1.6 L/mol, so our experiment yielded fairly accurate results.

The next method to determine molar volume was a stoichiometric approach. We found the mass of the reactant Mg to be 0.079g. Since the mol-to-mol ratio of Mg to H2 is 1:1, moles of hydrogen will be the same as moles magnesium. Dividing 0.079g by 24g (≈ 1 mol Mg), we find we have 0.0033 mol Mg, hence 0.0033 mol H2. Once again, using the same state of gas law, we can determine the volume of hydrogen at STP (V2 = 0.090L (738 torr / 760 torr) * (273K / 296K)) to be 0.081L. Dividing 0.081L by 0.0033mol, we find the molar volume of hydrogen to be 24.5L/mol.

The molar volume that we determined for hydrogen is slightly higher than what we found for oxygen (by 0.5L/mol). This is interesting, but looking back to the charts provided in the lab notebook, the true molar volume for hydrogen is 0.037L/mol greater than oxygen. Assuming that the experimental conditions for both the production of O2 and H2 were the same, this actual difference could account for our observed deviation.

The main source of error for this experiment is within the reactions themselves. Hydrogen and oxygen gas may have partially dissolved into the water itself, or may have not reacted in the original solution. In both cases, the results would be thrown off, resulting in a lower observed molar volume. This is because when calculating the volume of gas, the reading would be lower, since less water would have been displaced into the beaker. This volume would be multiplied in the state of gas laws, which would directly lower the calculated V2. Another source of error could be if the equipment was not working properly, and therefore a leak may develop which could seriously affect results because it would be impossible to accurately determine volumes and to equalize the pressures.