Effect of Molecular Weight on the Rate of Diffusion

Olive Kristianne C. Quicoy
Group 4 Sec. Y-5L

October 7, 2015

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A scientific paper submitted in partial fulfillment of the requirements in General Biology 1 laboratory under Ma’am Joan Christine O. Adajar, 1st sem., 2015-2016
ABSTRACT
The relationship of molecular weight and rate of diffusion was determined using three colored substances with different molecular weights, namely Potassium permanganate, Potassium dichromate and Methylene Blue. A water-agar gel in a petri dish was used to be able to observe the movement of the particles of each substance. Potassium dichromate had the fastest rate of diffusion among the three substances. Thus, the smaller the molecular weight, the faster the rate of diffusion.

INTRODUCTION
The movement of molecules from the area of higher concentration to the area of lower concentration is called diffusion (Mader & Windelspecht, 2013). Diffusion is observed when cooking pasta, a helium balloon deflates, drinking hot tea, and in many other situations in our daily lives, thus understanding it is important.
A lot of factors affect the diffusion of particles, such as time and molecular weight. Molecules of smaller mass diffuse faster than those with larger mass (Robinson and Hotzclaw, 1988). If this is so, then the smaller the molecular weight, the faster the rate of diffusion.
To observe how time and molecular weight affects the rate of diffusion, a medium that permits diffusion and substances that will allow easy observation as they diffuse is needed. Water, the universal solvent, permits diffusion and is readily available. To be able to observe the movement of the molecules in water, colored substances must be used. Another factor about the substances that has to be considered is the molecular weight of the substances. The substances to be used should be of different molecular weights. The substances used for this study are Potassium permanganate (KMnO4), Potassium dichromate (K2Cr2O7) and Methylene Blue. The molecular weights of the substances are 158 g/mole, 294 g/mole and 372 g/mole, respectively. KMnO4 is a purple substance, K2Cr2O7 is yellow and Methylene Blue has its color in its name.
Diffusion occurs quickly in water, and the movement of the molecules would be irregular. To be able to measure the rate of diffusion, a water-agar gel was used. Agar is a gelatinous substance derived from algae (Williams & Phillips, 2000). Agar forms a gel with water, thus allowing for measurement of the movement of the particles.
This study aimed to determine the relationship of molecular weight and the rate of diffusion. The specific objective is to describe the relationship between molecular weight and rate of diffusion.

MATERIALS AND METHODS To determine the effect of the molecular weight of substances on their rate of diffusion, three colored substances of different molecular weights were used, namely: Potassium permanganate (KMnO4), Potassium dichromate (K2Cr2O7) and Methylene Blue. To minimize the effect of movement of the container on the rate of diffusion of the substances, an agar-water gel was used. It was prepared on a petri dish, then three wells were made so that there will be one well per substance. A drop of each substance was then put into its assigned well at the same time, the petri dish was immediately covered afterwards. The set-up was then observed and drawn for comparison. The diameter of the colored area that each substance formed was measured every 3 minutes from 0 to 30 minutes. The measurements were recorded and the partial rates of diffusion were computed using the formula:Partial rates rp=di-di-1ti- ti-1 . The average rate of diffusion of each substance was also computed.
The set-up was again observed and drawn after 30 minutes for comparison. Graphs comparing the molecular weights and average rates of diffusion, and time and partial rates of diffusion were then plotted and analyzed.

RESULTS AND DUSCUSSION As seen in Table 1.2, Potassium dichromate and Methylene Blue had the same diameter – 6.0 mm – at 0 minute while Potassium permanganate had a diameter of 7.0 mm during the same time. After 30 minutes, Potassium permanganate, Potassium dichromate and Methylene Blue had the diameters of 9.0mm, 12.0mm and 10.0mm, respectively. Potassium dichromate had the largest increase in diameter among the three after 30 minutes.

Table 1.1. Diameters of the colored areas made by Potassium permanganate (KMnO4), Potassium dichromate (K2Cr2O7) and Methylene Blue in Agar Water Gel Time(minute) | Diameter (mm) | | Potassium permanganate(MW= 158 g/mole) | Potassium dichromate(MW= 294 g/mole) | Methylene Blue(MW= 372 g/mole) | 0 | 7.0 | 6.0 | 6.0 | 3 | 8.0 | 8.0 | 7.0 | 6 | 9.0 | 9.0 | 8.0 | 9 | 9.0 | 9.0 | 8.0 | 12 | 9.0 | 10.0 | 8.0 | 15 | 9.0 | 10.0 | 9.0 | 18 | 9.0 | 10.0 | 9.0 | 21 | 9.0 | 11.0 | 9.0 | 24 | 9.0 | 11.0 | 9.5 | 27 | 9.0 | 11.0 | 9.5 | 30 | 9.0 | 12.0 | 10.0 |

At 0 minute, the colors of Potassium permanganate, Potassium dichromate and Methylene Blue were purple, yellow and blue. After 30 minutes, K2Cr2O7 and Methylene Blue retained their colors while KMnO4 turned brown. The substances were not supposed to change in color during the experiment. The change in color of KMnO4 shows that the sample used was not pure. Figure 1.1 and Figure 1.2 shows an illustration of the set-up at 0 and 30 minutes. Figure 1.3 shows the actual set-up after 30 minutes.

Figure 1.1. Potassium permanganate, Potassium dichromate and Methylene Blue in Water-agar gel at zero minute

Figure 1.2. Potassium permanganate, Potassium dichromate and Methylene Blue in Water-agar gel after 30 minutes

Figure1.3. Water-agar gel set-up

After the diameter of the colored area of each substance was measured every 3 minutes, the partial rates of diffusion were computed individually. After the partial rates were computed, the average rate of diffusion for each substance was also computed. The partial rates of diffusion and the average rates of diffusion are shown in table 1.2.

Table 1.2. Rate of Diffusion of Potassium permanganate (KMnO4), Potassium dichromate (K2Cr2O7) and Methylene Blue in Agar Water Gel Time elapsed(minute) | Partial rates of diffusion (mm/min.) | | Potassium permanganate(MW= 158 g/mole) | Potassium dichromate(MW= 294 g/mole) | Methylene Blue(MW= 372 g/mole) | 3 | 0.33 | 0.67 | 0.33 | 6 | 0.33 | 0.33 | 0.33 | 9 | 0.00 | 0.00 | 0.00 | 12 | 0.00 | 0.33 | 0.00 | 15 | 0.00 | 0.00 | 0.33 | 18 | 0.00 | 0.00 | 0.00 | 21 | 0.00 | 0.33 | 0.00 | 24 | 0.00 | 0.00 | 0.17 | 27 | 0.00 | 0.00 | 0.00 | 30 | 0.00 | 0.33 | 0.17 | Average rate of diffusion(mm/min.) | 0.07 | 0.20 | 0.13 |

The computed average rates of diffusion of the three substances shows that Potassium dichromate had the fastest rate of diffusion, followed by Methylene Blue and Potassium permanganate. The difference in the rate of diffusion of the three substances is shown in Figure 1.4. The results does not clearly show the relationship between molecular weight and rate of diffusion since it is KMnO4 that has the smallest molecular weight (158 g/mole), followed by K2Cr2O7 (294 g/mole) and Methylene Blue (372 g/mole). The rates of diffusion is plotted against the molecular weight of the substances in Figure 1.5.

Figure 1.4. Partial rates of diffusion of Potassium permanganate (KMnO4), Potassium dichromate (K2Cr2O7) and Methylene Blue

158 g/mole - KMnO4
294 g/mole - K2Cr2O7
374 g/mole - Methylene Blue

Figure 1.5. Average rate of diffusion of Potassium permanganate (KMnO4), Potassium dichromate (K2Cr2O7) and Methylene Blue
SUMMARY AND CONCLUSION The relationship between molecular weight and rate of diffusion was determined. Potassium permanganate (KMnO4), Potassium dichromate (K2Cr2O7) and Methylene Blue were used for easy observation of the movement of the particles. A water-agar gel was used to be able to allow steady diffusion. The diameter of the colored area of each substance was measured every 3 minutes for 30 minutes. Results showed that K2Cr2O7, with the molecular weight of 294 g/mole had the fastest rate of diffusion instead of KMnO4 which had lower molecular weight of 158 g/mole. This is because the sample of KMnO4 used was contaminated, shown by the change in the color of the substance – from purple to brown – after 30 minutes. The reason for the contamination of the substance was unknown. Therefore, the smaller the molecular weight, the faster the rate of diffusion. To successfully come up with good results, it is recommended to use pure substances to make sure that the molecular weight of the substance is really the same as the theoretical one.
LITERATURE CITED
Holtzclaw, H.F. Jr. & Robinson, W.R. General Chemistry (8th ed.). Massachusetts: D. C.
Health and Company

Mader, S.S. & Windelspecht, M. (2013). Biology (11th ed.). New York, NY: McGraw-Hill

Williams, P.A. & Phillips, G.O. (2000). Handbook of Hydrocolloids. Cambridge: CRC
Press