The Effect of Molecular Weight on the Rate of Diffusion of Substances

Topics: Ammonia, Molecular diffusion, Diffusion Pages: 6 (2083 words) Published: March 3, 2013
The effect of molecular weight on the rate of diffusion was measured using two tests namely: the glass tube test and the agar-water test. The set-up of the glass tube test used two cotton balls of the same size. One cotton ball is moistened with hydrochloric acid (HCl) and the other one is moistened with ammonium hydroxide (NH4OH). The two cotton balls were inserted in both ends of the glass tube. NH4OH which has a lighter molecular weight (35.0459 g/mole) diffused with a faster rate (dave=20.25cm) as compared to HCl which has a greater molecular weight (36.4611g/mole) and diffused with dave=16.38 cm. A white ring of smoke formed closer to the heavier substance. The agar-water gel set-up used a petri-dish of agar-water gel with three wells on it. A drop of potassium permanganate (KMnO4) was put on one well, a drop of potassium dichromate (K2Cr2O7) was put on the other and a drop of methylene blue was put on the third well. Methylene blue which has the heaviest molecular weight of 374 g/mole occupied a small diameter of colored area which is 11mm. and had the slowest rate of diffusion which is 0.613 mm/min. Thus the lighter the molecular weight, the faster is the rate of diffusion.

As we open a household ammonia it will not take that long before the smell of it occupies the whole room. The gaseous molecules travel quickly and mix with the molecules in the air which makes it possible for people to immediately smell the ammonia as soon as it is opened. Such process is known to be the diffusion (Myers, Oldham and Tocci, 2006). Gases tend to diffuse rapidly with each other. It does not matter even if the air in a particular place is still because it will just take some minutes before the ammonia molecules occupy the place. During the process of diffusion, the substance moves from an area of higher concentration to an area of lower concentration. According to Myers, Oldham and Tocci (2006), particles of low mass diffuse faster than particles of high mass. According to Meyertholen (2007), some other factors are considered to have an effect in increasing or decreasing the rate of diffusion. Such factors include temperature or presence of energy, the distance, the barriers, the concentrations and many more. At a certain temperature, the smaller particle diffuses faster than the bigger one. This is because a bigger molecule requires greater force in able to move such particle, unlike the smaller particle which merely needs a lesser force for it to move (Meyertholen, 2007). With this study, we can derive into a hypothesis that “the rate of diffusion of a particle is inversely proportional to its size”, with that is the statement that “if molecular weight is involved in diffusion, then the substance with the lighter molecular weight will move faster”. The glass tube set-up was used to prove that the molecular weight of a substance affects its rate of diffusion. Two cotton balls of the same size, one was moisten with hydrochloric acid (HCl), other with ammonium hydroxide (NH4OH), were plugged into both ends of the glass tube. NH4OH has a lower molecular weight of 35.0459 g/mole while HCI has a molecular weight of (36.4611 g/mole). Together they will form ammonium chloride (NH4Cl) which will be the basis for measuring which of the two travels/diffuses faster. In order to identify the effect of molecular weight to the rate of diffusion of different substances, the water agar-gel set-up was used. Three wells are on the petri dish of agar-gel and three solutions are prepared, one drop of solution for every well. The solutions are potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7), and methylene blue. Through measuring the diameter of covered area of each solution for 30 minutes, rate of the diffusion were analyzed and compared. This study aimed to determine the effect of molecular weight to the rate of diffusion of different substances via the glass tube set-up and water agar-gel set-up. The specific...
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