The Effect of Molecular Weight on the Rate of Diffusion
of Potassium permanganate (KMNO4), Potassium
dichromate (K2Cr2O7) and Methylene blue ¹
The effect of molecular weight on the rate of diffusion was determined using the Agar-water Gel Test. One drop of potassium permanganate (KMNO4), potassium dichromate (K2Cr2O7) and methylene blue were used. Each substance has a respective molecular weight of 158 g/mol, 294g/mol and 374 g/mol. The diameters (in millimeter) were measured over time and the partial rates were computed. Results showed that as the time elapsed increases, the rate of diffusion decreases and the substance with the lowest molecular weight had the highest diffusion. Thus, the rate of diffusion of diffusion is inversely proportional with time and molecular weight.
Diffusion is the movement of molecules, atoms or ions of a substance across a membrane from a region of high concentration to a region of low concentration in a medium. The movement in diffusion is a net movement where any given particle could move in any direction at any particular time until equilibrium is reached (Morgan et al., 1969). At equilibrium, molecules continue to move back and forth but the net change on either sides of the membrane is zero because the particles are distributed evenly (Campbell et al., 2006). According to Sherman and Sherman (1989), the different rates of diffusion of a substance is due to the inherent heat energy of the molecules proportional dependent to temperature, electrical charges and arrangement of atoms or molecules, pressure, and molecular weight.
Past experiments using a glass tube and cotton balls moistened with hydrochloric acid (HCl) and ammonium hydroxide (NH4OH) showed that the rate of diffusion of each substance is dependent on their molecular weight. Furthermore, a hypothesis was formulated that if the molecular weight affects the rate of diffusion of substances, then a higher molecular weight would slow down the rate of diffusion.
The validity of the hypothesis formulated can be determined using a medium in which diffusion is observable and measurable. The agar-water gel is a good experimental material for this because any movement on the petri dish would not disturb and influence the rate of diffusion.
The main purpose of the study is to determine the effect of molecular weight on the rate of diffusion. Specifically, this study aims 1. to determine the rates of diffusion of potassium permanganate, potassium dichromate and methylene blue over time; and 2. to explain the possible principles behind the observed effect of molecular weight on the rate of diffusion.
The study was conducted at Room C-117 of the Biological Science Institute building, University of the Philippines Los Banos, Laguna on August 10, 2010 at 4 o’clock in the afternoon.
MATERIALS AND METHODS
To determine the effect of the molecular weight of a substance on the rate of diffusion, the Agar-water Gel Test was used. One drop of potassium permanganate (KMNO4), potassium dichromate (K2Cr2O7) and methylene blue were separately placed on the three wells of the agar gel plate. The molecular weights of the substances used were 158 grams per mole, 294 grams per mole and 374 grams per mole, respectively. The agar gel plate contained in a Petri dish was immediately covered after the drops had been placed. Diameters were measured in millimeter (mm) then tabulated and an illustration of the set-up at zero minute was drawn. Each substance’s diameter was measured at a regular three-minute interval for 30 minutes and a final illustration of the set-up was made.
After measuring the diameters of each substance, the partial rates of diffusion were computed using the formula:
Partial rate = di – di-1
ti – ti-1
The obtained values of the partial rates were tabulated and the average rates were computed by...
Cited: Campbell, N.A., et al. 2006. Biology: Concepts and Connections. 5th ed. Philippines: Pearson Education South Asia PTE. LTD. p. 81
Dickson, T.R. 1987. Introduction to Chemistry. 5th ed. Canada: John Wiley & Sons, Inc. p. 320
Morgan, D., et al. 1969. Biological Science: The Web of Life. Australia: Simmon Limited. p. 134
Sherman, I.W. and V.G. Sherman. 1989. Biology: A Human Approach. 4th ed. New York: Oxford University Press, Inc. p. 31
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