The effect of molecular weight on the diffusion rate

The effect of molecular weight on the diffusion rates of Potassium permanganate (KMnO4), Potassium dichromate(K2Cr2O7) and Methylene Blue^1

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Group 3 Sec. X-1L

February 6, 2012

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ABSTRACT

The effect of molecular weight on the diffusion of substances were tested through the use of agar-water gel with three wells placed in a petri dish. Drops of three different substances namely: Potassium permanganate (KMnO4), Potassium dichromate (K2Cr2O7) and Methylene Blue were placed in the wells of the agar-water gel in the petri dish. The diameter of the diffused substance was noted every 3 minutes for 30 minutes. The Potassium permanganate (KMnO4), which has the smallest molecular weight have the fastest average rate of diffusion compared to the other two substances with higher molecular weight. Thus, molecular weight affects the diffusion rate of substances. The higher the molecular weight the slower the diffusion rate is.

INTRODUCTION

Cell is the basic unit of life. It is enclosed by plasma membrane and is selectively permeable. Selective permeability is the function of the cell where it regulates the passage of specific molecules and materials that will enter and go out of it to maintain homeostasis. Homeostasis is the ability of the cell to seek and maintain a specific condition of equilibrium or stability within its internal environment in dealing with external exchanges. There are different substances that enter and go out of the cell. The process of crossing the plasma membrane can be classified as either passive or active transport (Calipjo et. al., 2010).

The substances that go in and out of the cell are made up of different molecules. “Molecule is two or more atoms held together by covalent bonds. Molecules have an energy type called heat or thermal motion. And this thermal motion might result to diffusion. Diffusion is the spontaneous movement of the molecules of a substance down its concentration gradient from a region where it is more concentrated to a region where it is less concentrated (Campbell et. al., 2009).” It is the tendency of molecules to spread out and occupy the vacant space where there is less concentration.

The heavier the molecular weight of a substance the slower is its diffusion rate. This was derived from the experiment using the NH3/HCl in a glass tube conducted in our class. Two cottons were soaked with the substances ammonium hydroxide (NH4OH) and hydrochloric acid (HCl) and were plugged at each end of the glass tube. The reaction between the gaseous molecules of the substance ammonia (NH3) from the substance ammonium hydroxide (NH4OH) and hydrochloric acid (HCl) formed the white smoke which is the evidence of diffusion. Among the four trials of the experiment, it is clearly shown in Table 1.1 that the diffusion occurred near the cotton plug with hydrochloric acid (HCl).

Table 1.1 Distance measurements of where the diffusion take place inside a glass tube plugged with two cotton balls at both sides soaked with two different substances namely ammonium hydroxide (NH4OH) and hydrochloric acid (HCl).
Trial Distance (cm) Total Distance Ratio (d) (D)

dHCl dNH3 1 15 24.5 39.5 0.38 0.62 1.63 2 10.5 29.5 40 0.26 0.74 2.85 3 12.5 29 41.5 0.30 0.70 2.33 4 14 26 40 0.35 0.65 1.86 Average ratio: 2.17cm
The distance of the white smoke from the cotton plug with hydrochloric acid (HCl) is smaller than the distance of the white smoke from the ammonium hydroxide (NH4OH). This was because the molecular weight of hydrochloric acid (HCl) is heavier than the molecular weight of ammonia (NH3) which makes it evident that the ammonia diffused at a faster rate.
This study aimed to determine the effect of molecular weight on the diffusion rates of different substances by observation. The specific objectives were:
1. to describe the effect of molecular weight on the diffusion of different substances with different molecular weight by the diameter it will form after thirty minutes of monitoring; and
2. to explain the possible reason on why other substances diffuse faster with the help of mean of the rates of diffusion.

MATERIALS AND METHODS In evaluating the effects of molecular weight on the diffusion rates, different substances with different molecular weights were tested in an agar-water gel. The agar-water gel was placed inside a petri dish. When the agar-water gel was already formed like a jelly, three holes, which were approximately 1.5 inch apart from each other, were formed at the middle. Three different substances with different molecular weights were obtained. These three different substances are namely: potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7) and methylene blue. Each substance is carefully dropped at its respective well in the agar-water gel in the petri dish. The substances were dropped with the help of a droplet to avoid the substance from dropping and scattering at the other part of the petri dish. The first substance that was dropped was the Potassium permanganate (KMnO4). Potassium permanganate (KMnO4) has the molecular weight of 158 g/mole and was dark violet in color. The second substance was Potassium dichromate (K2Cr2O7). Potassium dichromate (K2Cr2O7) has the molecular weight of 294 g/mole and was orange in color. And the last substance placed in its well was methylene blue. Methylene blue has the molecular weight of 374 g/mole and was dark blue in color. Right after placing the substances at their respective wells, the researchers observed the diffusion and measured the diameter of the substances every 3 minutes for thirty minutes.

After thirty minutes, partial rate of diffusion was computed by the measurements of the diameter of the colored part and using this formula:
FORMULA
After computing for the partial rates of diffusion, the researchers computed for the average rate of diffusion with the use of the computed partial rates of diffusion. With those computed results the researchers plotted the average rate of diffusion against the molecular weight of the substances and the partial rate of diffusion against the time elapsed.
RESULTS AND DISCUSSION As seen in Table 2 results showed that the different substances with different molecular weight have different diameter measurements in every time their diameter is measured. It was also seen that there were instances where the diameter measurements of the substances doesn’t change. This shown that the substance diffusion rate slows down after it was dropped in the well.

Also it was seen in the table that the potassium permanganate (KMnO4), which has the lightest molecular weight (158 g/mole), has the largest diameter (19mm) formed after thirty minutes. The substance with the second largest diameter (16mm) formed was potassium dichromate (K2Cr2O7), while Methylene blue, which has the heaviest molecular weight (374 g/mole), got the smallest diameter (11mm) after thirty minutes.

In Table 3, the partial rates of diffusion of the substances were shown. The average rates of diffusion were also seen in this table.
Table 2
Table 3
Figure 1
Figure 2
Figure 1 and 2 further shows the faster diffusion rate for the substance with the lowest molecular weight, potassium permanganate (KMnO4) is fastest.
SUMMARY AND CONCLUSION
The effect of molecular weight on the rate of diffusion of substances Potassium permanganate (KMnO4), Potassium dichromate (K2Cr2O7) and Methylene Blue was determined. These three substances were placed in the wells of agar-water gel inside a petri dish. After every three minutes within thirty minutes, the diameters formed by the diffused substances were measured.
Results showed that the Potassium permanganate (KMnO4), the substance with the lowest molecular weight (158 g/mole) and highest diameter (19mm) has the fastest average diffusion rate (0.43mm/min) and Methylene Blue, the substance with the highest molecular weight (374 g/mole) and smallest diameter (11 mm) has the slowest average diffusion rate(0.17 mm/min).
Therefore, molecular weight is a factor that affects the diffusion rate of a given substance. The higher the molecular weight of a substance the slower it will diffuse.

LITERATURE CITED
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Cited: Angeles, D.G., Dignos, G.B. and Ganaden, M.M. 2010. Chemistry. Quezon City, Philippines: Neo Asia Publishing Inc..p.229-235. . February 3, 2012.