To test the effect of molecular weight on the rate of diffusion, various experiments were performed. One of which is the glass tube test wherein cotton balls of the same size were moistened in two different substances (NH4OH and HCl). These cotton balls were plugged at each side of a glass tube. After some time, formation of a white ring occurred. The white ring, in fact, is a product of the reaction between the molecules of ammonia (NH3) and hydrochloric acid (HCl).
Results showed that NH3 which has a relatively lower molecular weight (MW NH3= 17.03 g/mol) than HCl (MW HCl= 36.46 g/mol) diffused at a faster rate (dave NH3 = 19.35 cm) compared to HCl (dave HCl= 16.18 cm). Another experiment was performed with the use of petri dish containing an agar-water gel with three wells. One drop of each substance (potassium dichromate, potassium permanganate, and methylene blue) was placed on each respective well.
In three-minute interval for 30 minutes, potassium permanganate, which has the lowest molecular weight (MW= 158 g/mol), displayed the largest diameter (d= 17.5 mm) and diffused with the fastest rate (0.52 mm/min). On the other hand, methylene blue, which has the highest molecular weight (MW= 374 g/mol), exhibited the smallest diameter (d= 10.5 mm) and diffused with the slowest rate (0.18 mm/min). Hence, the lower the molecular weight, the faster the rate of diffusion.
MATERIALS AND METHODS
Each group obtained a petri dish of agar-water gel with three wells. These wells were labeled: potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7), and methylene blue. At once, a drop of each substance was placed into each well respectively. Then, in three-minute interval for 30 minutes, the change in diameter (in mm.) of the colored areas was measured and recorded in Table 4.2. Moreover, the set-ups at zero minute and after 30 minutes were drawn for comparison in Figure 4.1 and 4.2 respectively.
The average rate of diffusion (in mm/min.) was calculated afterwards. This can be done by computing the partial rate of diffusion with the use of the equation:
Partial rate (rp) = (di – di -1)/( ti – ti-1)
Where: di= diameter of colored are at given time
di-1= diameter of colored are immediately before di
ti= time when di was measured
ti-1= time immediately before ti
Then compute the mean by adding all the calculated values divided by the number of values. All computed values were recorded and tabulated in Table 4.3. Finally, the average rate of diffusion of each substance against its molecular weight and the partial rate of diffusion of each substance against the time elapsed was plotted and given an interpretation in Figures 4.3 and 4.4 respectively. RESULTS AND DISCUSSION
Table 4.2 presents the change in diameter of potassium permanganate, potassium dichromate and methylene blue placed on a petri dish containing an agar-water gel at three-minute interval for 30 minutes. Using the table, it can be inferred that the substances with lower molecular weight diffuse faster than substances with higher molecular weight. For instance, potassium permanganate, based on the data given, has a molecular weight of 158 g/mole.
Its diameter, relative to the distance the colored areas occupied, became larger from 5 mm to 17.5 mm in 30 minutes. In addition, the diameter of potassium dichromate (MW= 294 g/mole) increased from 5 mm to 16 mm at same time interval. Moreover, methylene blue, with a molecular weight of 374 g/mole, had the smallest diameter increase having a diameter of 5 mm in 0 minute to 10.5 after 30 minutes. (TABLE 4.2)
Figure 4.1 and Figure 4.2 shows the diameter of colored areas in 0 minute and after 30 minutes respectively. Analyzing the figures, an increase in diameter of the colored areas was noticed. It was observed that the methylene blue displayed the smallest increase in diameter while potassium permanganate exhibited the largest increase. (FIG 4.1) (FIG 4.2)
It can be depicted in Table 4.3 that there is a relationship between the average rate of diffusion and molecular weight of potassium permanganate, potassium dichromate, and methylene blue. KMnO4 which has the lowest molecular weight attained the highest average rate of diffusion with 0.52 mm/min. K2Cr2O7, on the other hand, had 0.37 mm/min average rate of diffusion. Furthermore, methylene blue which has the highest molecular weight displayed the lowest average rate of diffusion with only 0.18 mm/min. (TABLE 4.3)
As seen in Figure 4.3, potassium permanganate, being the lightest among the three substances, obtained the highest average rate of diffusion (0.52 mm/min.) while methylene blue, being the heaviest, obtained the lowest average rate of diffusion (0.18 mm/min.). (FIGURE 4.3)
Lastly, in Figure 4.4, results showed that the partial rates of diffusion decreases as the time increases. Potassium dichromate, for example, had 1.17 mm/min. rate of diffusion in 3 minutes but only had 0.33 mm/min. rate of diffusion after 30 minutes. (FIGURE 4.4)
Relative to the hypothesis made, the agar-water gel test provided a concrete support on the correlation between the diffusion rate of a substance and its molecular weight. The two has an inversely proportional relationship such that if a substance has a low molecular weight, then it would have a high rate of diffusion and vice versa. SUMMARY AND CONCLUSIONS
The agar-water gel test was used to determine the effect of molecular weight on the rate of diffusion. A drop of potassium permanganate, potassium chromate, and methylene blue was placed on their respective wells at the same time. After 30 minutes, in three-minute interval, the diameter of each well was measured and recorded. Among the three, potassium permanganate exhibited the largest diameter (17 mm) while the methylene blue displayed the smallest measure (10.5 mm).
Likewise, potassium permanganate had the highest average rate of diffusion (0.52 mm/min.) and the methylene blue had the lowest (0.18 mm/min.). Based on the results of the experiment, the researcher found out that the molecular weight and the rate of diffusion of a substance is inversely proportional to each other given that the higher the molecular weight, the lower will be its rate of diffusion. For that reason, this conclusion agrees with the hypothesis formulated. Various factors may also affect the rate of diffusion of a substance.
One of which is the temperature such that as temperature increases, the amount of energy available for diffusion is increased. Thus, the higher the temperature, the higher will be the rate of diffusion (Meyertholen, 2007). Moreover, the rate of diffusion may also be affected by concentration gradient, diffusion distance, surface area, and permeability. It is recommended to conduct an experiment to better comprehend the relationship between the said factors and the rate of diffusion of a substance. LITERATURE CITED
Chang,R., Overby, J., 2008. General Chemistry: The Essential Concepts (6th ed.). New York, USA: Mc-Graw Hill. P. 162. Duka, I-M. A., Diaz, M.G., Villa, N.O., 2009.Biology I Laboratory Manual: An Investigative Approach (9th ed.). UPLB: p.34-39 Enger, E. D., Kormelink, J. R., Ross, F. C., Smith, R. J. 1991. Concepts in Biology (6th ed.). USA: Wm. C. Brown Publishers. p. 34-35. Meyertholen, E. (n.d.) Diffusion. . Accessed September 7, 2013.
Miller K.R., Levine, J. 1991. Biology. Needham, Massachusetts: Prentice Hall. p. 99-100 Robinson, W. R., Odom, J. D. and Holtzclaw, H. F. Jr. 1992. Chemistry: Concepts and Models. Lexington, Massachusetts: DC Heath. p. 282. Zumdahl, S. S.1992. Chemical Principles. Lexington, Massachusetts: DC Heath and Company. P. 155-156
Diffusion is the process by which molecules of a substance move from areas of higher concentration of that substance to areas of lower concentration (Miller and Levine, 1991). It is also defined as the net movement which is the movement of molecules in one direction minus the movement of molecules in the opposite direction (Enger, 1991). In other words, net diffusion is the movement of molecules along the concentration gradient.
Diffusion can also be used to describe the mixing of gases. For example, when a small amount of pungent-smelling ammonia is released at the front of a classroom, it takes some time before everyone in the room can smell it, because time is required for the ammonia to mix with the air (Zumdahl, 1992). One of the main factors that affect the rate of diffusion is the molecular weight. If a mixture of gases is placed in a container with porous walls, diffusion of the gases throughout the walls occur.
The lighter gases diffuse through the small openings of the porous walls more rapidly than the heavier ones (Robinson, 1992). In accordance to this, an experiment was performed to see the relationship between the molecular weight and rate of diffusion of a substance. The glass tube set–up was done by plugging in two cotton balls which are uniform in size at the opposite ends of a glass tube.
One cotton ball was soaked in ammonium hydroxide (NH4OH) and the other with hydrochloric acid (HCl). Results showed that molecules of NH4OH diffused at a faster rate than the molecules of HCl. Thus, the hypothesis will be: a molecule with a lower molecular weight diffuses in a faster rate than a molecule with a higher molecular weight.
This study aimed to:
1. Further explain the effect of molecular weight to the rate of diffusion. 2. Identify other factors that may affect the rate of diffusion. This study was conducted at Room C-127, Institute of Biological Sciences, University of the Philippines – Los Banos, Laguna on September 2, 2013.