The Effect of Molecular Weight on the Diffusion Rate of Substances

Topics: Petri dish, Agar plate, Diffusion Pages: 5 (1274 words) Published: August 16, 2013
The Effect of Molecular Weight on the Diffusion Rate in Substances

Nadjah B. Hadji Amer
Gladys A. Oliveros
Rochelle R. Yaῆez
Jan Remon B. Caliguid
Group 2- Sec. W78-1

August 14, 2013

A scientific paper submitted in partial fulfilment of requirements in Biology 101.1 under Prof. Kimverly Hazel Coronel, 1st sem, 2013-2014


The effect of molecular weight on the rate of diffusion was assessed using the agar-water gel test. The agar-water gel set up was composed of a six petri dish of agar-water gel containing two wells. Pinch of potassium permanganate (KMnO4) and methylene blue (C16H18N3SCl) were simultaneously introduced to each dish and measured the distance and its rate. Methylene blue, having the largest molecular weight, displayed the smallest diameter 15.67 and diffused at the slowest rate (0.78mm/min). Next is potassium permanganate with a diameter of 13.33 mm and rate of diffusion of 2.54mm/min. The fastest is the potassium permanganate with 13.33 mm diameter and diffusion rate of 2.54 mm/min. Thus, the higher the molecular weight, the slower the rate of diffusion.


Diffusion is a process wherein molecules of gases collide and interact as a result of random motion. This eventually leads to the uniform distribution of the molecules of the involved gases throughout the system (Nave, 2008). Diffusion is a net movement of particles from an area of high concentration to low concentration (Traverso, 2004). Several factors may affect the rate of diffusion of substances. These factors include the particle size or the molecular weight of the substance, the temperature in the system, the concentration difference of the substances, the diffusion distance, the surface area, and the permeability of the barrier. The larger the particle, the greater the force needed to move the particle. Thus, at a certain temperature, a smaller particle diffuses faster than a larger one (Meyertholen,2012) The hypothesis is to determine the effect of molecular weight on the rate of diffusion of substances with respect to time via the agar-water gel test. Specifically, it aimed to: 1. Identify the factors that affect the diffusion rate of substances; and 2. Explain the effect of molecular weight on the diffusion rate of substances.

A petri dish containing water-agar gel was used as a medium of diffusion. Two solutions of different molecular weight were used: potassium permanganate (KMNO4), a red solution with a molecular weight of 158 g/mole and methylene blue, a blue solution with a molecular weight of 320 g/mole. Each substrate was placed carefully noting so that the crystals are spread equally over the agar and not too close to the edge of the dish. We made three replicates for each substrate and get the average value. The diameter of the colored area was measured immediately after adding the substance to the agar plate and records the measurement in millimiters (mm) for 0 minutes. In regular 15minutes interval, the diameters of the colored zone around each substrate were measured and record both the time (in minutes) and diameters in Table 1. It was measured up to one hour. After gathering the data, the rate of diffusion for the three substances was computed using the formula:

Rate= Final diameter – Initial Diameter
The data on diffusion rate (mm/min) versus molecular weight (g/mole) was plotted. The data of two substances (potassium permanganate and methylene blue was also plotted to see the difference. RESULT AND DISCUSSION

TABLE1. Diffusion Rate of Methylene Blue and Potassium Permanganate Methylene Blue(MW:320 g/mole)| Potassium Permanganate(MW: 158 g/mole)| Time(min)| Distance (mm)| Rate (mm/min)| Time(min)| Distance (mm)| Rate (mm/min)| 0| 15.67| 0| 0| 13.33| 0|

15| 20| 0.29| 15| 36.67| 1.56|
30| 28| 0.26| 30| 39| 0.78|
45| 36| 0.17| 45| 46| 0.16|
60| 40| 0.06| 60| 48.67| 0.04|

The size of the...

References: CITED
Edition. McGraw-Hill, USA. p. 208.Meyertholen, E. (n.d.). Diffusion. Retrieved on July 30, 2012 from
Nave, R. 2008. Diffusion and Osmosis. Retrieved on July 30, 2012 from 
Silberberg, M. S. 2000. Chemistry 2nd Edition. McGraw-Hill, USA. p. 205.
Traverso, M. 2004. Diffusion and Concentration Gradients. Retrieved on July 31, 2012from dynamic.htm
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