The Effect of Molecular Weight on the Diffusion Rate of Substances1

Selina Rabin S. Gualberto Group 2 Sec. I – 2L

August 1, 2012

_____________________ 1A scientific paper submitted in partial fulfillment of the requirements in General

Biology I under Prof. Kevin Labrador, 1st sem., 2012-2013.

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ABSTRACT

The effect of molecular weight on the diffusion rate

of substances was determined using the glass tube test and the agar-water gel test. In the glass tube test, two cotton plugs were soaked with hydrochloric acid (HCl) and ammonium hydroxide (NH4OH) and were inserted into both ends of the glass tube. NH4OH. The substance (NH4OH) with a lighter molecular weight of 35.0459 g/mol diffused at a faster rate, resulting in the formation of a white ring around the tube closer to the side of the substance with the larger molecular weight of 36.4611 g/mol (HCl). In the agar-water gel test, drops of potassium permanganate, potassium dichromate and methylene blue were simultaneously placed in separate wells in an agar plate. Methylene blue, with the largest molecular weight, had the smallest diameter (8.5 mm) and had the slowest diffusion rate (0.05 mm/min). Thus, the larger the molecular weight, the slower the rate of diffusion.

INTRODUCTION

Diffusion is a process wherein molecules of gases collide and interact as a result

of random motion. This eventually leads to the uniform distribution of the molecules of

the involved gases throughout the system (Nave, 2008). Diffusion is a net movement of

particles from an area of high concentration to low concentration (Traverso, 2004).

Several factors may affect the rate of diffusion of substances. These factors

include the particle size or the molecular weight of the substance, the temperature in the

system, the concentration difference of the substances, the diffusion distance, the surface

area, and the permeability of the barrier. The larger the particle, the greater the force

needed to move the particle. Thus, at a certain temperature, a smaller particle diffuses

faster than a larger one (Meyertholen, n.d.).

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The effect of molecular weight on the diffusion rate of gases can be observed with

the glass tube set-up. It involves simultaneously placing cotton plugs, separately

moistened with two solutions, on opposite ends of a glass tube. The set-up is observed

until a white ring forms around the tube near the end of the substance with the larger

molecular weight. In the experiment, hydrochloric acid (HCl) and ammonium hydroxide

(NH4OH) were used for easier comparison of the diffusion rates, since there is a

significant difference in the molecular weights of the substances (HCl = 36.4611 g/mol;

NH4OH = 35.0459 g/mol).

The agar-water gel set-up involves simultaneously placing drops of different

solutions in separate wells in an agar plate and measuring their diameters at regular time

intervals. In the experiment, the solutions used were potassium permanganate (KMnO4),

potassium dichromate (K2Cr2O7) and methylene blue. The three substances possess

distinct colors, making it easier to differentiate and measure their diameters.

This study aimed to determine the effect of molecular weight on the rate of

diffusion of substances with respect to time via the agar-water gel test.

Specifically, it aimed to:

1. identify the factors that affect the diffusion rate of substances; and

2. explain the effect of molecular weight on the diffusion rate of substances.

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MATERIALS AND METHODS

In determining the effect of molecular weight on the diffusion rate of substances,

the glass tube and agar-water gel set-ups were used. For the glass tube set-up, the

materials used were a 30-cm glass tube, an iron stand with an iron ring, rubber bands,

cotton plugs, and NH4OH and HCl solutions. For the agar-water gel set-up, the materials

used were an agar plate with wells, potassium permanganate (KMnO4), potassium

dichromate (K2Cr2O7) and methylene blue. All the materials were acquired from the

Dispensing Laboratory of the College of Science and Mathematics, University of the

Philippines Mindanao Campus, Davao City.

Glass Tube Set-up

The 30-cm glass tube was held level in place on the iron ring using the rubber

bands. Two cotton plugs were separately soaked in ammonium hydroxide (NH4OH) and

hydrochloric acid (HCl) and simultaneously inserted into both ends of the glass tube.

Four replicates were made. After some time, white rings of smoke formed on the inside

of the tubes. The positions were marked.

The distances from the HCl and NH4OH plugs to the white rings were measured

and recorded in centimeters (cm). The average distance from the substances to the

position of the white smoke was calculated by summing up the total distances and

dividing it by the number of replicates. A table comparing the measurements of the

distances was plotted and analyzed.

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Agar-Water Gel set-up

A Petri dish containing agar-water gel with three wells was used as the medium

for diffusion. Three solutions were used: potassium permanganate (KMnO4), potassium

dichromate (K2Cr2O7) and methylene blue. These substances have different colors and

molecular weights; KMnO4 has a purple color and has a molecular weight of 158 g/mol,

K2Cr2O7 is a yellow solution and has a molecular weight of 294 g/mol, and methylene

blue is a blue solution with a molecular weight of 374 g/mol. A drop of each solution was

separately placed in the wells, and the Petri dish was immediately covered to avoid air

from affecting the diffusion rates of the solutions.

The diameters (in millimeters) of the colored areas were measured at three-minute

time intervals, beginning at t0 = 0 minutes and ending at t11 = 30 minutes. The average

rate of diffusion was calculated by taking the average of the computed partial values. The

partial diffusion rate was calculated using the following formula:

Partial rate (rp) = di – di-1 ti – ti-1

where: di = diameter of the colored area at a given time

di-1 = diameter of the colored area immediately before di

ti = time when di was measured

ti-1 = time immediately before ti

The computed values were tabulated, and the mean of the partial rates of each

substance was calculated. The average diffusion rates of the substances were plotted

against their corresponding molecular weight. The partial rates of each substance were

also plotted at time intervals. These tables and graphs were analyzed.

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RESULTS AND DISCUSSION

Table 1 shows the distances (in cm) of the smoke rings formed from the HCl- and

NH4OH-soaked cotton plugs inserted into each end of the glass tube. The white smoke is

ammonium chloride (NH4Cl), which is the product formed from the reaction of NH4OH

with HCl. It is observed that the distance from the smoke ring is generally closer to the

HCl end of the tube, ranging from 8 to 16.5 cm, compared to that with NH4OH, which

ranges from 13 to 17.5 cm. The average distances of the smoke from NH4OH and HCl

are 15.55 cm and 11.625 cm, respectively. This shows that the reaction occurred near the

HCl end of the tube.

The size of the particle is inversely proportional to the rate of diffusion of the

substance (Silberberg, 2000). Hydrochloric acid has a molecular weight of 36.4611 g/mol,

and Ammonium hydroxide has a molecular weight of 35.0459 g/mol. Since HCl has a

larger molecular weight, it diffuses slower than NH4OH. Since NH4OH diffused faster

than HCl, it was able to reach the HCl end of the glass tube faster than the HCl travelled

towards the NH4OH end. The formation of the white smoke indicated that the NH4OH

molecules have reached the HCl molecules and were already reacting to form NH4Cl.

It can be inferred from Table 1 that the rates of diffusion of substances are

inversely proportional to their corresponding molecular weights. The hypothesis, “if

molecular weight affects the diffusion rates of substances, then the higher the molecular

weight of a substance, the slower the rate of diffusion”, is tested with the next set-up.

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Table 1. Distance of the smoke ring from the HCl and NH4OH ends of the glass tube.

Trial Distance, d (cm)

Total distance, D Ratio

dHCl dNH4OH dHCl/D dNH4OH/D NH4OH/HCl

1 11 13 24 .4583 .5417 1.1818

2 8 18 26 .3077 .6923 2.25

3 16.5 17.5 34 .4853 .5147 1.0606

4 11 13.7 24.7 .4453 .5547 1.2455

Table 2 shows the data for the agar-water gel set-up. The diameters of the areas

covered by the three substances (KMnO4, K2Cr2O7 and methylene blue) were measured

(in mm) at three-minute intervals, starting at 0 minutes and ending at 30 minutes. The

average diameters were calculated, and KMnO4 had the largest diameter (15.818 mm),

followed by K2Cr2O7 (14.455 mm), and methylene blue had the smallest diameter (7.909

mm). Potassium permanganate has a molecular weight of 158 g/mol, potassium

dichromate is 294 g/mol, and methylene blue is 374 g/mol. With this, it can be observed

that methylene blue, being the heaviest, had the smallest diameter covered. Meanwhile,

potassium permanganate, being the lightest, had the largest diameter covered after 30

minutes. The diameters covered show the diffusion of the substances. These results

support the hypothesis that the higher the molecular weight, the slower the rate of

diffusion.

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Table 2. Diameters of KMnO4, K2Cr2O7 and methylene blue on the agar plate at three-minute intervals.

Time (minute)

Diameter (mm) Potassium

Permanganate, KMnO4 (158 g/mole)

Potassium Dichromate, K2Cr2O7

(294 g/mole)

Methylene Blue (374 g/mole)

0 8 8 7 3 10 10 7 6 12 12 7 9 13 12 7 12 16 15 8 15 17 15 8.5 18 18 16 8.5 21 18 17 8.5 24 20 18 8.5 27 21 18 8.5 30 21 18 8.5

Average 15.818 14.455 7.909

Figure 1 shows the agar-water gel set-up after 30 minutes. It can be observed that

the colored areas have different diameters. The darkest and largest colored area is the

potassium permanganate. The yellow-colored area is potassium dichromate, and the blue

and smallest one is methylene blue.

Figure 1. The agar-water gel set-up after 30 minutes.

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Table 3. Partial and average rates of diffusion of KMnO4, K2Cr2O7 and methylene blue on the agar plate at three-minute intervals.

Time (minute)

Partial rates of diffusion (mm/min)

Potassium Permanganate (158 g/mol)

Potassium Dichromate (256 g/mol)

Methylene Blue (375 g/mol)

3 0.6667 0.6667 0 6 0.6667 0.6667 0 9 0.3333 0 0 12 1 1 0.3333 15 0.333 0 0.1667 18 0.333 0.3333 0 21 0 0.3333 0 24 0.6667 0.3333 0 27 0.3333 0 0 30 0 0 0

Average 0.4333 3.333 0.5

Table 3 shows the partial and average diffusion rates of the three substances. The

average diffusion rates show that potassium permanganate diffuses the fastest, followed

by potassium dichromate, and, lastly, methylene blue. Figure 2 shows the line graph of

the partial diffusion rates in Table 3. Several times, the diffusion rate became zero at

some intervals. According to Chang (1998), the rate of diffusion of a substance is

inversely proportional to the molecular weight. This is because the larger the particle, the

more energy is required to move the particle.

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0

0.2

0.4

0.6

0.8

1

1.2

3 6 9 12 15 18 21 24 27 30

Part

ial r

ate

of d

iffus

ion

(mm

/min

)

Time elapsed (min)

PotassiumPermanganate (158g/mol)

PotassiumDichromate (256g/mol)

Methylene Blue (375g/mol)

Figure 2. A line graph of the partial rates of diffusion of KMnO4, K2Cr2O7 and methylene blue against the time elapsed.

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SUMMARY AND CONCLUSION

The effect of molecular weight of the rate of diffusion was determined with the

agar-water gel set-up. Drops of potassium permanganate (KMnO4), potassium

dichromate (K2Cr2O7) and methylene blue were simultaneously placed in separate wells

in the agar plate. The diameters of the substances were measured at three-minute intervals,

from 0 minutes to 30 minutes.

The results showed that at the end of 30 minutes, KMnO4 had the largest diameter

covered at 21 mm, followed by K2Cr2O7, which covered 18 mm. Methylene blue had the

least covered area at 8.5 mm. The rates of diffusion of KMnO4, K2Cr2O7 and methylene

blue are 0.43 mm/min, 0.33 mm/min, and 0.05 mm/min, respectively.

The results acquired coincide with the hypothesis that if molecular weight affects

diffusion rates, then the larger the molecular weight, the slower the diffusion rate of the

substance. Aside from this, however, other factors such as temperature, concentration, the

diffusion medium used also affect the rate of diffusion.

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LITERATURE CITED

Chang, R. 1998. Chemistry 6th Edition. McGraw-Hill, USA. p. 208.

Meyertholen, E. (n.d.). Diffusion. Retrieved on July 30, 2012 from http://www.austincc.edu/~emeyerth/diffuse2.htm

Nave, R. 2008. Diffusion and Osmosis. Retrieved on July 30, 2012 from http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/diffus.html

Silberberg, M. S. 2000. Chemistry 2nd Edition. McGraw-Hill, USA. p. 205.

Traverso, M. 2004. Diffusion and Concentration Gradients. Retrieved on July 31, 2012 from http://www.chemistry.wustl.edu/~courses/genchem/Tutorials/Kidney/ dynamic.htm