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The Effect of Molecular Weight on the Rate of Diffusion of Substances

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The effect of molecular weight on the rate of diffusion was measured using two tests namely: the glass tube test and the agar-water test. The set-up of the glass tube test used two cotton balls of the same size. One cotton ball is moistened with hydrochloric acid (HCl) and the other one is moistened with ammonium hydroxide (NH4OH). The two cotton balls were inserted in both ends of the glass tube. NH4OH which has a lighter molecular weight (35.0459 g/mole) diffused with a faster rate (dave=20.25cm) as compared to HCl which has a greater molecular weight (36.4611g/mole) and diffused with dave=16.38 cm. A white ring of smoke formed closer to the heavier substance. The agar-water gel set-up used a petri-dish of agar-water gel with three wells on it. A drop of potassium permanganate (KMnO4) was put on one well, a drop of potassium dichromate (K2Cr2O7) was put on the other and a drop of methylene blue was put on the third well. Methylene blue which has the heaviest molecular weight of 374 g/mole occupied a small diameter of colored area which is 11mm. and had the slowest rate of diffusion which is 0.613 mm/min. Thus the lighter the molecular weight, the faster is the rate of diffusion.

As we open a household ammonia it will not take that long before the smell of it occupies the whole room. The gaseous molecules travel quickly and mix with the molecules in the air which makes it possible for people to immediately smell the ammonia as soon as it is opened. Such process is known to be the diffusion (Myers, Oldham and Tocci, 2006). Gases tend to diffuse rapidly with each other. It does not matter even if the air in a particular place is still because it will just take some minutes before the ammonia molecules occupy the place. During the process of diffusion, the substance moves from an area of higher concentration to an area of lower concentration. According to Myers, Oldham and Tocci (2006), particles of low mass diffuse faster than particles of high mass. According to Meyertholen (2007), some other factors are considered to have an effect in increasing or decreasing the rate of diffusion. Such factors include temperature or presence of energy, the distance, the barriers, the concentrations and many more. At a certain temperature, the smaller particle diffuses faster than the bigger one. This is because a bigger molecule requires greater force in able to move such particle, unlike the smaller particle which merely needs a lesser force for it to move (Meyertholen, 2007).

With this study, we can derive into a hypothesis that “the rate of diffusion of a particle is inversely proportional to its size”, with that is the statement that “if molecular weight is involved in diffusion, then the substance with the lighter molecular weight will move faster”. The glass tube set-up was used to prove that the molecular weight of a substance affects its rate of diffusion. Two cotton balls of the same size, one was moisten with hydrochloric acid (HCl), other with ammonium hydroxide (NH4OH), were plugged into both ends of the glass tube. NH4OH has a lower molecular weight of 35.0459 g/mole while HCI has a molecular weight of (36.4611 g/mole). Together they will form ammonium chloride (NH4Cl) which will be the basis for measuring which of the two travels/diffuses faster. In order to identify the effect of molecular weight to the rate of diffusion of different substances, the water agar-gel set-up was used.

Three wells are on the petri dish of agar-gel and three solutions are prepared, one drop of solution for every well. The solutions are potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7), and methylene blue. Through measuring the diameter of covered area of each solution for 30 minutes, rate of the diffusion were analyzed and compared. This study aimed to determine the effect of molecular weight to the rate of diffusion of different substances via the glass tube set-up and water agar-gel set-up. The specific objectives were: 1. To identify factors that affect the diffusion rate of substances. 2. To describe the effects of molecular weight and other factors like time with rate of diffusion of the different solutions.

Glass Tube Set-up
To measure the rate of diffusion through its molecular weight a glass tube set-up was used. A glass tube is placed into a fume hood. Two cotton balls of the same size was obtained. One cotton ball was soaked in hydrochloric acid (HCl) and the other was soaked in ammonium hydroxide (NH4OH). Both cotton balls were plugged at the same time on both ends of the glass tube. After some minutes, a white ring of smoke was observed to form near the hydrochloric acid (HCl). The distance between the white ring of smoke and the cotton ball with ammonium hydroxide (NH4OH) was recorded and the same was done with the distance between the white ring of smoke and the cotton ball with hydrochloric acid (HCl). Three other glass tube set-ups were obtained, measured and recorded. The total distance of set-ups 1 to 4 were calculated by adding the distances of the two cotton balls from the white ring of smoke. The ratio between the (HCl) and (NH4OH) was also measured by dividing the ratio of each substance from its distance from the white ring of smoke. Average distance was also calculated by adding the total distances of the four set-ups. The calculated data were organized into table. Water Agar-Gel Set-up

To measure the rate of diffusion with the factors of molecular weight and time the water agar-gel set-up was used. A petri dish with water agar-gel was set up. The water-agar gel has three wells on it. Three solutions with three different molecular weights were distributed to the three wells. The red solution is the Potassium Permanganate (MW 158 g/mole), the yellow one is the Potassium dichromate (MW 294 g/mole) and the blue solution is the Methylene blue (MW 374 g/mole). After filling each well with a particular solution all of the same time, the diameter of the colored area for each well was measured and recorded for zero minute. For thirty minutes with a regular three-minute interval, the diameter of the colored area for each well was measured. The records of all measurement were organized into table. The average rate of diffusion for each solution was obtained by computing first the partial rate of diffusion at each interval. The formula for computing the partial rate is …. Where….

The partial rates were totaled and then divided to the total number of observations in order to get the average of diffusion. The average of diffusion of each solution is then graphed. The partial rates of distribution were also put into graph for analysis and interpretation.

The table 1 shows the distance of HCl and NH4OH from the formed white ring of smoke for each trial. The white smoke that was formed is actually ammonium chloride (NH4CI) which is the product between the reaction of gasses of both HCl and NH4OH. The four trials clearly showed the same pattern where the white ring of smoke formed closer to HCI which has a heavier molecular weight. This observation only reveals that NH4OH which has a lighter molecular weight diffused faster than HCl. The ring of smoke’s distance from NH4OH ranges from 19-22 cm while the ring of smoke’s distance from HCI ranges from16-17 cm. Computing the average distance of NH4OH-to-ring of smoke and HCI-to-ring of smoke made it clearer that NH4OH diffused with the faster rate of 20.25 cm. Hydrochloric acid (HCl) has a molecular weight of 36.4611g/mole and ammonium hydroxide (NH4OH) has a molecular weight of 35.0459 g/mole. HCI is heavier in nature and a slower rate of diffusion is identified with it. NH4OH have the opposite situation. The fact that the white ring of smoke appeared loser to the HCI manifests that the NH4OH traveled faster and in longer distance as compared to HCI. Based on the data on table1, we could deduce that the rate of diffusion of molecule comes in different measurement for different molecular weights.

Table 2
The table2 shows result of how fast Potassium permanganate, Potassium dichromate and methylene blue diffused after thirty minutes with the interval of three minutes per measurement. The rate of measurement was assessed by measuring the diameter of the colored area for each solution. Potassium permanganate with the lowest molecular weight (158 g/mole) among the three covered the biggest diameter of colored area, while Methylene blue with the highest molecular weight of 374 g/mole recorded to have the smallest diameter of colored area. The observation revealed that the greater molecular weight means a lower diameter of covered area. Table 3

Table 3 shows the trend of rate of diffusion of each solution. In here Potassium dichromate (has the second lowest molecular weight) and not Potassium permanganate (has the lowest molecular weight) appears to have the fastest diffusion rate. With the said observation, the hypothesis that “the higher the molecular weight, the smaller the diffusion” appeared to be not always right. That maybe because of the presence of some factors like the: A) inaccurate measurements, B) unequal amount of solutions put and C) for not dropping the solutions all at the same time. The results of the average rate of diffusion are the basis of knowing which of the three solutions diffused the fastest. Methylene blue (has the greatest molecular weight) consistently appears to have the slowest diffusion rate. Figure 1

Supposedly a line that forms a letter “C” will be formed but the said image was failed to achieve. Instead of the Potassium permanganate, the Potassium dichromate appears to have the fastest diffusion rate of the three. The aforementioned factors possible created this kind of trend. There are inconsistencies that are present. Figure 2

Figure 2 evidently showed Potassium permanganate diffused the fastest for the first minutes but as it approaches the 30-minute time, the diffusion rate went slower and slower. But clearly it is still the Potassium permanganate that covered the biggest diameter among the three. Unlike the others, Potassium dichromate can be seen to have a consistent diffusion rate. Methylene blue which has the greatest molecular weight of the three shows a trend that is very inconsistent and very slow when it comes to the rate of diffusion. The hypothesis that “the lighter molecular weight, the faster the diffusion rate” is true for Methylene blue but not for the other two.

Two set-ups are used to test the effect of molecular weight and time to the rate of diffusion. One is through the use of Glass tube set up where diffusion rate was measured through the distance of white ring of smoke from hydrochloric acid (HCl) and ammonium hydroxide (NH4OH). The other is through placing a drop three different solutions to three wells in a petri dish of water agar-gel. The solutions are Potassium permanganate, Potassium dichromate and Methylene blue. With the first set-up, NH4OH that has the smaller molecular weight of the two was measured to travel faster than the HCI. NH4OH has an average distance of 20.25 as compared to HCI which has 16.38. With the second set-up, Methylene blue which has the biggest molecular weight appeared to have the smallest average rate of diffusion (.613) and smallest diameter of area covered.

This manifests that Methylene blue has the slowest rate of diffusion. Potassium dichromate appeared to have a faster rate of diffusion (.776) when it is supposedly be the Potassium permanganate (.7476), but then the two do not differ much from each other. The two set-up, though not one-hundred percent, prove that the smaller the molecular weight, the higher rate of diffusion. Unequal amount of solution for the three wells, uncoordinated way of putting the solutions into the wells and some inside and outside factors played a big role in proving the hypothesis to be right. More accurate results will be produced if the experiment will be done with the absence of other factors that negatively affect the result.

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