The effect of molecular weight on the rate of diffusion was assessed using two tests: the glass tube test and the agar-water gel test. In the glass tube set-up, two cotton plugs soaked in two different substances (HCl and NH4OH) were inserted into the two ends of the glass tube. The substance with the lighter molecular weight value (NH4OH, M = 35.0459 g/mole) diffused at a faster rate (dAve = 25.8cm), resulting in the formation of a white ring around the glass closer to the side of the heavier substance (HCl, M = 36.4611 g/mole; dAve = 10.8 cm). 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 were simultaneously introduced to each well. Methylene blue, having the largest molecular weight, displayed the smallest diameter (18 mm) and diffused at the slowest rate (0.3668 mm/min.). Thus, the higher the molecular weight, the slower the rate of diffusion. INTRODUCTION
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. 3
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 validity that the molecular weight of a substance has an effect on its rate of diffusion was derived from the glass tube set-up. Two cotton balls of identical size were moistened with hydrochloric acid (HCl) and ammonium hydroxide (NH4OH) which were placed simultaneously in each end of the glass tubing. The molecular weight of HCl (36.4611 g/mole) is larger compared to the 4
value of the molecular weight of NH4OH (35.0459 g/mole) and both react to form ammonium chloride (NH4Cl), a solid product, which make the two substances suitable for comparison on which substance diffuses faster inside the glass tube. 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 in three different wells on a petri dish with agar-water gel. The three substances are dyes and possess different colors which make...