The purpose of this experiment was to determine the molar mass of carbon dioxide (CO2) experimentally.
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Determining the molar mass of a gas Experiment date: 13/10/10 Performed by: Hannah Chan & Alexander Forman The purpose of this experiment was to determine the molar mass of carbon dioxide (CO2) experimentally. A simple calculation using the periodic table would provide the correct answer for the molar mass of carbon dioxide, however, one can also conduct an experiment and try to reach the accepted value. Introduction: The ideal gas law equation(PV = nRT) defines the relationships between pressure (P), volume (V), number of moles (n), and temperature (T) for any ideal gas sample. R is the ideal gas constant, defines as 0.0821 L · atm/K · mol. Therefore P must be expressed in atmospheres (atm), V in liters (L), n in moles (mol), and T in Kelvin (K). Almost all experimental conditions correspond with the ideal gas law equation. Only when the gas pressure is several atmospheres or higher does the behaviour deviate from the equation. In order to calculate the molar mass of CO2, one must first be familiar with this equation. Hypothesis: It was expected that the mass would be approximately 44 g mol-1. Materials: * Volumetric flask, 100cm3, dry with stopper * Scale with accuracy of three decimal places.
filled with water 157.2 g ± 0.1 Room temperature 21 °C ± 0.5 °C Atmospheric pressure 750 mmHg Density of air under conditions of experiment 0.00199 g cm-3 Table 17a 15 °C 17 °C 19 °C 21 °C 23 °C 25 °C 740 mmHg 0.00119 0.00119 0.00118 0.00117 0.00116 0.00115 750 mmHg 0.00121 0.00120 0.00119 0.00119 0.00118 0.00117 760 mmHg 0.00123 0.00122 0.00121 0.00120 0.00119 0.00119 770 mmHg 0.00124 0.00123 0.00123 0.00122 0.00121 0.00120 780 mmHg 0.00126 0.00125 0.00124 0.00123 0.00122 0.00122 Table 17b (Density of air (g cm-3) at different temperatures and pressures) Analysis: Before the Ideal gas equation was used to calculate the molar mass of CO2, some calculations were done. 1) Calculating the volume of the flask. From table 17a the mass of the flask with both air and water was read. The mass of the flask with air was subtracted from the mass of the flask with water, leaving only the mass of the water. Knowing the density of water (1 g cm-3), the volume of the flask was deduced. (Mass of flask + Water ) - Mass of flask = mass of water = volume of flask. 157.222 g - 48.303 g = 108.919 g = 108.919 cm3 2)
Density = mass/volume. d = 0.18661 g/108.919 cm3 · 10-3 = 1.71329 g m-3 3) In step (4) why were you told to remove the delivery tube slowly? It was to prevent the carbon dioxide escaping from the flask. 4) Why was a less accurate balance adequate for weighing the flask full of water? It was more adequate because the mass of water is much larger than both CO2 and air, increasing the uncertainty and removing the need for a large number of decimal places. Errors & improvements: The molecular mass of carbon dioxide is known to be approximately 44 g mol-1, however, in this experiment the molar mass of CO2 turned out to be only 41.9 g. There are several reasons for this error. The most likely being the concentration of CO2 in the flask. Some of the carbon dioxide would escape before the stopper has sealed the flask. The CO2 from the generator might have not been completely pure. Another reason might have been a systematic error caused by the scale leading to incorrect values, or simply an uncertainty error by rounding too much. Conclusion: The relationship between the actual amount (44 g mol-1) and the calculated amount (41.9 g mol-1) was significant. The procedure of the experiment would not be functional in finding an unknown gas. ?? ?? ?? ?? Alexander Forman 2IB 06/10/2010
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