The Effect of Varying Molecular Weights on the Rate of Diffusion

The effect of molecular weight on the rate of diffusion was verified by the 2 tests: the glass tube setup and the water agar-gel setup. In the glass tube setup, two cotton balls were soaked in the solutions of hydrochloric acid (HCl) and ammonium hydroxide (NH4OH) and were simultaneously placed on both ends of the tubing.NH4OH had a lighter molecular weight of 35 g/mole which diffused at a faster rate of 24.8 cm and formed a white smoke near the HCl end that had the molecular weight of 36 g/mole. The water agar-gel setup was made up of a petri dish containing the gel with three wells. Drops of potassium permanganate (KMnO4), potassium dichromate(K2Cr2O7) and methylene Blue were concurrently placed in the wells. Methylene Blue had the smallest diameter which diffused at the slowest Rate of 0.13 mm/min since it has the largest molecular Weight. Thus, the larger the molecular weight, the slower It will diffuse.

INTRODUCTION The particles in different substances are never still because of its capability to keep moving and bumping into each other all the time.  This type of movement is called diffusion (Kent, 2000). Diffusion is the process by which substances spread from regions of high concentration to the regions of low concentration until there is equilibrium. According to Rowland (1992), the greater the difference in concentration is then the faster the rate of diffusion of the substance. This may then serve as one of the factors that affect the rate of diffusion. Some factors include the temperature, polarity of the molecules and its molecular weight. Molecular weight could then be defined as the mass of a molecule that is relative to the mass of atom. Satake (1997) also proposed that molecular weight is proportional to the rate of diffusion.

Through this study, it will be determined whether Satake’s proposal is true or not. Based on Garg’s studies on the Multiple Effect Diffusion Solar Still during 1987, he proposed that the lower values of molecular weight of a gas, the rate of diffusion will be higher. Therefore a lighter gas such as hydrogen must be put in the chamber since it will provide higher values of diffusion rates. If the total pressure in the chamber is also decreased, then the diffusion rate can also be increased. The hypothesis that “If the molecular weight affects the rate of diffusion, then the higher the molecular weight, the slower the diffusion” was derived from the glass tube set-up. Two cotton balls were soaked instantaneously in a solution of hydrochloric acid (HCl) and ammonium hydroxide (NH4OH) and were placed inside each end of the glass tube. The molecular weight of HCl (36 g/mole) is greater than the molecular weight of NH4OH (17 g/mole). From this, an observation was made regarding the diffusion of gases , therefore it give rise to the hypothesis of the effect of molecular weight to the rate of diffusion After it was derived, it was then verified by means of the agar-water gel test.

The set-up included the use of potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7) and methylene blue that were sinked in the three wells in the agar-water gel. Throughout the testing of the hypothesis, the rate of diffusion was measured using its diameter in the time range of 30 minutes. This study aimed to determine the effect of molecular weight on the rate of diffusion of sbustances via the agar-water gel test. Specifically, it aimed to: 1. To establish weight and time as two factors affecting the rate of diffusion 2. To determine the effects of the two factors on the rate of diffusion. The study was performed at one of the laboratories in the Institute of Biological Sciences Wing C Building, University of the Philippines Los Banos Campus on August 14,2012. MATERIALS AND METHODS On establishing the hypothesis, the glass tube setup was utilized. A two feet glass tube was placed horizontally on a ring stand as shown in Fig.1. Two identical cotton balls were then simultaneously soaked in a solution of hydrochloric acid (HCl) and the other in ammonium hydroxide (NH4OH). Once soaked, the cotton balls were separately placed at both ends of the glass tubing.

Afterwards, it was observed that a white smoke appeared in the glass tubing and the position was marked. The distances (in cm) form both ends of the glass tubing until the position where it was marked, were measured. The data was then recorded into a table. The values of the total distances and ratios were calculated then. The hypothesis was then to be tested by the agar water gel set-up. A petri dish of agar water gel with 3 wells was used as the medium of diffusion. Agar water gel was used as the medium since it has a framework of solid particles which trap water in their apertures. Drops of each of three solutions were then put into each well. Specifically, these are potassium permanganate (KMn04), which was red and had the molecular weight of 158 g/mole, potassium dichromate (K2Cr2O7), a yellow solution with a molecular weight of 294 g/mole and methylene blue, a solution that was blue and had a molecular weight of 374 g/mole. The diameter at 0 minute was then measured and recorded. After a fixed three minute interval within the thirty minutes, the diameter of the three wells was then measured and noted into a table.

The average rate of diffusion (in mm/min) was also computed by averaging the partial rates of diffusion. The partial rates were calculated with the use of the formula: Partial rate: (Rp)= di-di-1 Where di=’afa From this, all of the computed values were put into a table. These values were then plotted, evaluated and construed. Results and Discussion: Table 4.1 illustrates the distances of the white smoke formed from the reaction of hydrochloric acid (HCl) and ammonium hydroxide (NH4OH)in the glass tubing. The white smoke was a product of the chemical reaction of the two solutions, which is called ammonium chloride (NH4Cl). It was noted that the white smoke was formed drastically nearer to the HCl end with a distance ranging from 6.2 to 20 cm in against to the distances of NH4OH ranging from 16 to 31.5 cm. The reason was that HCl has a molecular weight of 37 g/mole while NH3 had a molecular weight of 35 g/mole. HCl was heavier compared to NH4OH, which makes it diffuse slower than the other solution.

And since NH4OH was lighter in terms of molecular weight, it must have diffused faster towards the HCl end of the glass tube. The ammonium chloride then formed near the HCl was a sign that the NH4OH have reacted with HCl coming from the opposite side. The hypothesis derived from the glass tube set-up was then tested by the use of the agar water gel set-up. In Table 4.2 shows that methylene blue had the smallest diameter of 9 mm after the time interval of 30 minutes. While methylene blue had the smallest, potassium permanganate had the largest diameter of 18 mm then potassium dichromate of 16 mm after 30 minutes. This observation showed that the hypothesis aforementioned was true because with the data given, the idea of having a molecular weight will slow down its rate of diffusion is evident. In Figures 4.1 and 4.2, there was a significant change in terms of the size of the diameter of each well from zero minute to thirty minutes. The diameter of each well was then recorded with potassium permanganate having the largest diameter of 18 mm from the three wells, while potassium dichromate with 16 mm and methylene blue with 9mm.

In Table 4.3, the computed values of the average and partial rates of diffusion were calculated and noted. It showed potassium permanganate got the highest value of its average rate of diffusion. We must bear in mind that potassium permanganate (KMnO4) had the lowest molecular weight while methylene blue had the highest molecular weight; however, methylene blue diffused the slowest with 0.13 mm/min out of the 3 substances. Clearly, there is a relationship between the molecular weight of a substance to its average rate of diffusion. This relationship is presented in Figure 4.3. The average rate of diffusion is still different from the partial rates of diffusion. From Table 4.2, it can be clearly seen how much the diameter of each well has grown within the time interval of 30 minutes. But from these measurements alone, the hypothesis could not be solely proved for it needs more data to support it. So after recording the diameters in the table, partial rates of diffusion were then calculated with the use of the formula aforementioned. Potassium permanganate (KMnO4) had continuously decreased in terms of the partial rates of diffusion, and so did potassium dichromate.

It was observed that methylene blue did not decrease after every 3 minute interval , compared to potassium dichromate (K2Cr2O7) and potassium permanganate (KMnO4). The pattern concerning the rate of diffusion could not be exactly determined, yet there was continuous change. To also have a better understanding of the partial rates of diffusion, refer to Figure 4.4. In this, the time served as the independent variable in the graph. Time also had an effect to the rate of diffusion because in the first few minutes, the rate of diffusion was higher compared to the last six minutes of the 30 minute interval. Therefore, time could be established as a factor that affects the rate of diffusion. SUMMARY AND CONCLUSIONS The effect of molecular weight on the rate of diffusion was verified by the water agar gel set-up.

A drop of potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7) and methylene blue were placed instantaneously on the three wells in the petri dish provided. The diameter of the wells at 0 minute to 30 minutes were then measured and recorded for every 30 minutes. After the 30 minute mark, methylene blue had the smallest diameter of 9 mm, potassium dichromate with 16 mm and potassium permanganate with 18 mm had diffused the slowest with its are methylene blue rate of diffusion of 0.13 mm/ min and potassium permanganate with 0.43 mm/ min. With this data, the hypothesis could then be accepted. However, there were still sources of errors that when avoided, could possibly result to a better understanding of the study. Some sources of errors could possibly be that the wells in the gel had cracks in it and that the angle of the dropper could have made the drop fall right into the center of each well. It is recommended that the test would be performed in a place with better conditions, such that other factors, like temperature or amount of substance would affect the outcome of the results.