The Effect of Varying Molecular Weights on the Rate of Diffusion

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The Effect of Varying Molecular Weights on the Rate of Diffusion of Substances

August 22,2012


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.

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...
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