The Correlation between the Diffusion Rate of a Substance and its Molecular Weight ABSTRACT
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...