Glass Tube Experiment

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The Relationship between Molecular Weight and the Rate of Diffusion of Substance

Anthony Earl D. Cristobal
Group 1 Sec. U-7L

16 October 2014

The relationship between the molecular weight and the diffusion of the substance was determined using two laboratory experiments. In the first experiment, a horizontally placed glass tube was set. Two cotton balls were soaked in hydrochloric acid (HCl) and ammonium hydroxide (NH3). At the same time, the cotton balls were inserted at both ends of the tube. Several seconds later, a white cloud formed where the gases of the substances met, but nearer to the side of the hydrochloric acid, which has heavier molecular weight than ammonium hydroxide. In the second experiment, a
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A drop of potassium permanganate, potassium dichromate and methylene blue were introduced to those wells. Every three minutes the diameters those drops are measured and recorded for 30 minutes. Since methylene blue has the highest molecular weight (374g/mole) among the three solutions, methylene blue has the slowest rate of diffusion and has the shortest diameter.
Materials and Method
Glass Tube Experiment A glass tube was horizontally placed on an iron stand. Two cotton balls of identical size were soaked in hydrochloric acid and ammonium hydroxide and were plugged simultaneously to the separate ends of the tube. After several seconds, a white cloud inside the tube was observed and the measure from both ends to the cloud was recorded.

Agar-water gel Experiment A petri dish containing agar-water gel with three holes was given. Each of those wells are dropped with potassium permanganate (158 g/mole), potassium dichromate (294 g/mole) and methylene blue (374g/mole). The diameters of the three drops were recorded at a three minute interval for thirty
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This also supports the hypothesis that molecular weight affects the rate of diffusion.
Table 5: Line graph of the diameters of the substances in a three minute interval for thirty minutes

Summary and Conclusion According to the experiments made and data gathered, the results strongly support the hypothesis. HCl, having the greater molecular weight, diffused slower than NH3, was closer to the white cloud formed inside the tube than the side of NH3. Potassium permanganate, which has the least molecular weight among the three substances in the agar-water experiment, obtained the largest diameter while the methylene blue, which has the largest molecular weight, got the shortest. Because of these experiments, we can therefore conclude that there is a relationship between the molecular weight and the rate of diffusion. They are inversely proportional. Therefore the higher the molecular weight, the slower the rate of

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