Effect of Molecular Weight in the Rate of Diffusion

• Jarrel Dean A. Yecyec

ABSTRACT

The effect of molecular weight in the rate of diffusion was determined by placing potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7), and methylene blue in a petri dish of agar-water gel with three wells. After a drop of each in well the petri dish was covered then, the diameter of the colored area was measured with three minute interval. It was measured ten times. Methylene blue has the lowest molecular weight but, Methylene blue showed the longest diameter. Thus, Molecular weight affects the rate of diffusion the higher the mass the slower the rate of diffusion.

INTRODUCTION

According to the experiment of diffusion, two feet glass tube was fasten horizontally to a ring stand. After moistening two cotton balls of the same size, one with hydrochloric acid and the other with ammonium hydroxide it is plugged to one end and the other end. White smoke was observed inside the tube. When you measure the distance from the cotton to the white ring, ammonium hydroxide exhibits the longer distance.

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Diffusion demonstrates random motion that proceeds from a region of higher concentration to lower concentration. Diffusion it is when molecules try to achieve equilibrium. Diffusion of gases always happens gradually, it takes a relatively long time. For example, when you open a bottle of perfume inside a room it will take time before a person smell it in the other side of the room. This is explained by the numerous collisions of molecules while moving from one end to another. The root-mean-square speed is a way to estimate molecular speed. We can estimate how fast a molecule move, on the average, at any temperature. By interpreting the formula it shows that the heavier the gas, the slowly its molecules move.

Molecular weight it is the mass in grams of one mole of molecules of formula units of a substance. It is also called as molecular mass. The hypothesis was arrived based on previous observation, the hypothesis is if molecular weight affects the rate of diffusion then the higher the mass the slower the rate of diffusion.

Agar is a polymer made up of subunits of galactose, it is also a component of some cell walls. When dissolved in boiling water and cooled, agar looks gelatinous. Potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7), and methylene blue are the compounds that will be used in this experiment.

This study aimed to determine the effect of molecular weight to the rate of diffusion. The specific objectives were.

1. To describe the effect of diffusion
2. To explain why heavier molecules tend to be slower than lighter molecules.

MATERIALS AND METHODS

A petri dish of agar-water gel with three wells with a diameter of 5 millimeter was prepared. Potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7), and methylene blue were obtained. The color of potassium permanganate is red-violet, potassium dichromate is yellow, and methylene blue is blue this is based on observation. One drop of each prepared solution was then placed into each well and then the petri dish should be immediately covered, a dropper was used. A ruler was used for measuring the diameter of the colored area of each substance. The set up was then recorded at a regular three-minute interval for a half hour, for every interval the colored area was measured and recorded in table 4.2. A phone was used in recording the time.

After thirty minutes, the colored area was measured and recorded. The zero minute and the thirty minute were compared. A graph comparing the three solutions was then plotted and analyzed.

Partial rate of diffusion was calculated at each interval. The formula for the partial rate of diffusion is final minus initial diameter of colored area divided by the final minus initial time measured. The average rate of diffusion (mm/min.) will be calculated and graphed.

RESULTS AND DISCUSSION

As seen in Table 1 results showed that Potassium permanganate diffuse faster than the other two. There are times that they are constant showed in Figure 1. The diameter increases as time increases but there are points that there is no change in the diameter. Potassium permanganate with the lowest molecular weight is the fastest to diffuse and Methylene blue with the highest molecular weight is the slowest to diffuse.

As seen in Table 2 Potassium permanganate has the highest average rate of diffusion, it is moving 0.37 millimeters per minute. It is the fastest compared to others. Figure 2 completely showed the comparison of average rate of diffusion.

The hypothesis if the molecular weight affects the rate of diffusion then the higher the mass the slower the rate of diffusion is accepted. It is supported by the root-mean-square speed because you can compute for the speed of a molecule and by the experiments.

Table 1. The diameter of the three solutions at a regular three-minute interval for thirty minutes.

Figure 1. A line graph comparing the three solutions and showing the Effect of time in the diameter of the colored area

Table 2. The partial rates of diffusion of the three solutions.

Figure 2 Comparison of the partial rates of diffusion of the three solutions.

SUMMARY AND CONCLUSION

The effect of the molecular weight in the rate of diffusion was determined. One drop of each prepared solution was placed into each well and the diameter was measured at a regular three minute interval for a half hour. The average rate of diffusion was computed and supported the experiment. The data we gathered is credible because there is no source of error.

The results showed that Potassium permanganate exhibits fast diffusion compared to others. Therefore, molecular weight affects the rate of diffusion, the heaver the compound the slower the rate of diffusion.

I recommend this experiment. add more compounds with different molar weight and different kind of samples too. If you are dealing with gases try to compute the theoretical speed of a molecule by using the root-mean-square speed and compare it with the one you observed.

LITERATURE CITED

Dorland, W.A.M. 2012.Agar, Dorland's Medical Dictionary. Retrieved January 17, 2013

Zumdahl, S.S 1998. Chemical Principles. 3^rd edition. Houghton Mifflin. A37.

Chang, Raymond. Chemistry. 6^th edition. New York: McGraw-Jill. p. 183-185

Duka, A.I.M, Diaz, Q.M.G, Villa, O.N. 2009. Biology 1 laboratory manual: An investigative approach. Ninth edition. UP Los Banos, College, Laguna, Philippines.

No author. Diffusion and Osmosis. <http://www.biologycorner.com/bio1/notes_diffusion.html>