The Effect of Different Molar Mass on the Diffusion on Substances

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The Effect of Different Molar Mass on the Diffusion on Substances

Lunar-maius A. Gaerlan
Group 2 Sec. X – 9l

August 15, 2012


The effect of molecular weight on the rate of diffusion was assessed using agar-water gel test. The agar-water gel set up was composed of a petri dish of agar-water gel containing three wells. Drops of potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7) and methylene blue(C16H18N3SCl) were simultaneously introduced to each well. Methylene blue, having the largest molecular weight, displayed the smallest diameter (11 mm) and diffused at the slowest rate (0.20 mm/min.). Next is potassium dichromate with a diameter of 24 mm and rate of diffusion of 0.30 mm/min.. The fastest is the potassium permanganate with 19 mm diameter and diffusion rate of 0.47 mm/min.. Thus, the higher the molecular weight, the slower the rate of diffusion.

A substance in the gaseous or liquid state consists of molecules or atoms that are independent, rapid, and random in motion. These molecules frequently collide with each other and with the sides of the container. In a period of time, this movement results in a uniform distribution of the molecules throughout the system. This process is called diffusion (Everett and Everett, n.d.). Diffusion occurs naturally, with the net movement of particles flowing from an area of high concentration to an area of low concentration. Net diffusion can be restated as the movement of particles along the concentration gradient. According to Meyertholen (n.d.), there are several factors which may affect the rate of diffusion of a substance. These factors include the size of the particle or the molecular weight of the substance, temperature or availability of energy in the system, difference in concentrations inside the system, diffusion distance, and if the system involves a membrane or barrier, surface area of the barrier, and the barrier’s permeability. The greater the concentration of a substance in an area of a system entails that the frequency of particles colliding with each other is higher, causing the particles to “push” each other at a faster rate. These collisions are due to the high molecular velocities associated with the thermal energy “powering” the particles (Nave, 2008). At a given temperature, a smaller particle is said to diffuse at a faster rate than a larger one. This is because the larger the size of a particle, a greater amount of force is said to be required to move the particle (Meyertholen, n.d.). With the same amount of energy, a smaller particle can be pushed faster than a larger particle. Thus, the hypothesis of the study is that the rate of diffusion is inversely proportional to the size of the particle. That is, a smaller particle will diffuse faster than a larger one. The agar-water gel test was used to assess and verify the effect of the molecular weight on the rate of diffusion of different substances. The set up involved the introduction of one drop of potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7), and methylene blue (C16H18N3SCl) in three different equidistant wells on a petri dish with agar-water gel. The three substances are dyes and possess different colors which make them easily identifiable and suitable for measurement of the diameter of the drops within a period of 30 minutes. This study aimed to assess the influence of molecular weight on the rate of diffusion of potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7) and methylene blue (C16H18N3SCl) with respect to time via the water-agar gel test. Specifically, it aimed to:

1. Identify a recurring pattern among the behaviors of diffusion of the substances in relation to their molecular weight; and 2. Elucidate the principle behind the observed behavior of the diffusion of the substances. The study was conducted in the Biology Laboratory of the Institute of Biological Sciences, University of the Philippines Los Baños Campus, Los...
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