1A scientific paper submitted in partial fulfilment of the requirements in General Biology

I laboratory under Miss Janece Polizon, 1st sem., 2014-2015.

The Effect of Molecular Weight on the Rate of Diffusion of Substances1

Group 1 Sec. UV-3L

October 20, 2014

Kathleen ArquizaSticky Note

Kathleen ArquizaSticky Note

2

ABSTRACT

The effect of molecular weight on the rate of diffusion was

determined by using two set-ups: the glass tube test set-up and the agar-

water gel set-up. In the glass tube test set-up, two cotton balls were soaked

in two different substances (HCl and NH4OH) and were plugged in the two

endings of the glass tube respectively. The substance with the lighter

molecular weight (NH4OH MW = 35.05 g/mole) diffused at a faster rate

(dAve. = 21.75 cm), resulting to the formation of white smoke closer to the

cotton ball with the heavier molecular weight (HCl MW = 36.458 g/mole;

dAve = 13.24 cm). In the agar-water gel set-up, each drop of potassium

permanganate (KMnO4 MW = 158 g/mole), potassium dichromate

(K2Cr2O7 MW = 294 g/mole) and methylene blue (MW = 374 g/mole) was

placed into each well simultaneously. Methylene blue, having the largest

molecular weight, has the smallest diameter (10 mm) after 30 min. and the

slowest to diffuse (average rate of diffusion = 0.20 mm/min.) while

potassium permanganate, having the lightest molecular weight, has the

largest diameter (12 mm) and diffused at the fastest rate (0.27 mm/min.).

Thus, the molecular weight and the rate of diffusion of a substance have an

inverse relationship.

INTRODUCTION

When you are in a room where some people smoke tobacco, you will notice that

the smoke coming from the tobacco does not stay close to the smokers; it moves throughout

the room until the whole place became smoky and the smell of tobacco is everywhere. This

event illustrates diffusion.

Diffusion is the process by which substances spread from the regions of high

concentration to the lesser concentrated regions (Campbell, 1987) caused by the natural

tendency of a substance to spread uniformly in the dispersion medium (Rastogi, 1997).

This movement is made possible by the Brownian movement (Mehrer, 2007). The particles

of the substance, like ions and molecules, show random thermal movements; they move

down a concentration gradient. The greater difference in concentration, the steeper

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concentration gradient and it will have faster diffusion rate. The ions and molecules still

move randomly but on average even when the diffusion process stops; they move equally

in all directions (Rowland, 1992). Diffusion of substances may be affected by some factors

such as time and molecular weight (Duka, et al., 2009). Thus, the hypothesis of the study

is that the rate of diffusion and the molecular weight have an inverse relationship. That is,

the substance with lighter molecular weight will diffuse at a faster rate than the substance

with the heavier molecular weight.

The validity that the diffusion rate is inversely proportional to the molecular weight

of a substance was derived from the glass tube test set-up. Two cotton balls of the same

size were moistened with two substances, hydrochloric acid (HCl), which has the heavier

molecular weight (36.458 g/mole), and ammonium hydroxide (NH4OH), with the lighter

molecular weight (35.05 g/mole), and then introduced to the opposite endings of the glass

tube to test which will diffuse faster. Because of the reaction of the substances resulting to

the formation of a solid product, ammonium chloride (NH4Cl), the set-up become more

suitable for the test to determine which will diffuse faster.

The agar-water gel set-up was used to determine and verify the effect of molecular

weight on the diffusion rate of substances. To easily identify the substances, three dyes

were used, namely potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7)

and methylene blue. A drop of each solutions (dyes) will be introduce to their respective

well in the set-up. The diameter of each will be measured within a period of 30 minutes.

This study aimed to determine the effects of molecular weight on the diffusion rate

of substances. The specific objectives were:

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1. to determine the various factors that affect the diffusion rate of substances; and

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

The study was conducted at the laboratory room C-127 in the Institute of Biological

Sciences Building, University of the Philippines Los Baos, Laguna, on October 13, 2014.

MATERIALS AND METHODS

In determining the rate of diffusion of substances, first, the glass tube test was used.

A glass tube was fastened to a ring stand as seen in Figure 1. Two cotton balls of the same

size were prepared. Under the fume hood, using fine forceps, each cotton balls were

carefully and simultaneously soaked into two different substances, one with hydrochloric

acid (HCl) and the other with ammonium hydroxide (NH4OH). One end of the glass tube

was plugged with one cotton ball and the other with another cotton ball. This was done

simultaneously. Four set-ups were assembled, three served as replicates.

After some time, a white smoke inside the glass tube was observed and its position

was marked. The distance (in cm) from each cotton to the marked position was measured

and was recorded and tabulated.

In the second experiment, to determine the rate of diffusion of substances, the agar-

water gel set-up was used. A petri dish of agar-water gel with three wells was obtained as

seen in Figure 2. The gel has three wells of equal sizes (4 mm each).

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iron ring

iron stand

glass tube

cotton plugs

HCl NH4OH

Figure 1. The glass tube test set-up

6

Figure 2. The agar-water gel set-up

petri dish

wells

agar-water gel

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Three different solutions with distinct color and different molecular weight were

obtained. Violet potassium permanganate (KMnO4), yellow potassium dichromate

(K2Cr2O7) and methylene blue that is blue has a molecular weight of 158 g/mole, 294

g/mole, and 374 g/mole respectively.

A drop of each solution was carefully and simultaneously placed into each well in

the agar water gel set-up. After that, the petri dish was immediately covered and the initial

diameter (in mm) of each colored area was measured. The measurements were recorded as

the diameter in zero minute.

At a regular three-minute interval for thirty minutes, the diameter of each colored

area was measured and recorded.

The partial rates of diffusion were then computed using the formula:

Partial rate (rp) =

Where: di = diameter of colored area at a given time

di-1 = diameter of colored area immediately before di

ti = time when di was measured

ti-1 = time immediately before ti

The computed values were tabulated and the average of each was calculated. Two

graphs were provided; the first graph was to see the effect of the molecular weight on the

di - di-1 ti - ti-1

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average rate of diffusion (in mm/min.), and the second graph was to see the effect of time

elapsed (in min.) on the partial rate of diffusion (in mm/min.).

RESULTS AND DISCUSSION

As seen in Table 1, results showed that the distances measured between the smoke

ring and the ammonium hydroxide, ranging from 21 cm to 22.50 cm, are longer than the

measured distances of the smoke ring and the hydrochloric acid, ranging from 10.50 cm to

14.75 cm. The observed smoke ring, ammonium chloride (NH4Cl), was the product of the

reaction of the two substances.

After computing the average distances, it shows that HCl-to-smoke ring average

distance (dAve = 13.24 cm) is lesser than the NH4OH-to-smoke ring average distance (dAve

= 21.75 cm). The result depicts that the initial reaction of HCl and NH4OH took place

nearer the cotton ball with HCl.

Since the HCl has a molecular weight of 36.458 g/mole and NH4OH has a

molecular weight of 35.05 g/mole, HCl should diffuse slower than NH4OH, as observed in

the experiment. NH4OH has a faster rate that is why it reached the other side faster than

HCl reaching its opposite side of the glass tube. This resulted in the formation of the smoke

ring as the indication that the molecules of NH4OH already met the molecules of HCl and

reacted with it.

Based on the data in Table 1, we could say that the measured rates of diffusion of

a series of molecules are proportional to their molecular weights.

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Table 1. Distance of the smoke ring from the hydrochloric acid and ammonium

hydroxide at the opposite side of the glass tube.

Trial

Distance (cm)

(d) Total

Distance

(D)

Ratio

dHCL dNH3 dHCL

D

dNH3

D

NH3

HCL

1 13.70 22.20 35.90 0.382 0.618 1.62

2 14.00 21.00 35.00 0.400 0.600 1.50

3 10.50 21.30 37.80 0.278 0.722 2.60

4 14.75 22.50 37.25 0.396 0.604 1.53

Average 13.24 21.75 36.49 0.364 0.636 1.81

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Table 2 contains the data obtained in the second experiment, which the agar-water

gel was been used. From 0 minute, the diameter of each well with different solutions were

measured and recorded from time to time, with three minutes interval, for thirty minutes.

Potassium permanganate with a molecular weight of 158 g/mole has the largest diameter

(12 mm), as shown in Figure 4, which means that it diffused at a fastest rate while the

methylene blue with a molecular weight of 374 g/mole, the heaviest among the three

solutions, diffused at the slowest rate. This was also shown in Figure 5. These observations

support the formulated hypothesis if the molecular weight affects the rate of diffusion of

substances, then the higher the molecular weight, the slower the diffusion.

In Table 3, the partial rates were calculated and tabulated, and the mean of each

solutions partial rates of diffusion was computed. The fast diffusion of potassium

permanganate was observed early after 3 minutes, with a partial diffusion rate of 0.67

mm/min, compared to the other solutions. There was a 0.07 mm/min difference between

the solution with the lightest molecular weight, potassium permanganate and the solution

with the heaviest molecular weight, methylene blue. The effect of time on the computed

partial rate of diffusion of the solutions were shown on Figure 6. Generally, diffusion rates

slowed down as the diffusion process progressed.

After the results were analyzed, all the data and observations supported the

aforesaid hypothesis. The molecular weight and the rate of diffusion of a substance have

inverse relationship. This means that the larger the molecular weight, the slower the rate of

diffusion, and vice versa. This occurs because of the size of the particle of a substance;

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Table 2. Diameter of potassium permanganate, potassium dichromate and methylene blue

drops on the wells of agar-water gel for 30 minutes, measured with three minutes

interval.

Time

(minute)

Diameter (mm)

Potassium

permanganate

(MW 158 g/mole)

Potassium

dichromate

(MW 294 g/mole)

Methylene

Blue

(MW 374 g/mole)

0 4 4 4

3 6 5 5

6 7 6 5

9 7 7 5

12 7 7 6

15 8 8 7

18 10 9 7

21 11 10 9

24 12 10 9

27 12 11 10

30 12 11 10

Average 8.73 8 7

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Figure 3. Potassium permanganate, potassium dichromate and methylene

blue drops at zero minute.

Figure 4. Potassium permanganate, potassium dichromate and methylene

blue drops after 30 minutes.

Potassium permanganate

Potassium dichromate

Methylene blue

Potassium permanganate

Potassium dichromate

Methylene blue

13

0

0.05

0.1

0.15

0.2

0.25

0.3

Potassium permanganate(MW 158 g/mole)

Potassium dichromate(MW 294 g/mole)

Methylene Blue(MW 374 g/mole)

Ave

rage

rat

e o

f D

iffu

sio

n (

mm

/min

.)

Molecular Weight (g/mole)

Figure 5. A bar graph showing the relationship of the average rate of diffusion of

potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7) and

methylene blue and their molecular weight.

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Table 3. Rate of diffusion of potassium permanganate, potassium dichromate and

methylene blue.

Time elapsed

(minute)

Partial rates of diffusion (mm/min.)

Potassium

permanganate

(MW 158 g/mole)

Potassium

dichromate

(MW 294 g/mole)

Methylene

Blue

(MW 374 g/mole)

3 0.67 0.33 0.33

6 0.33 0.33 0.00

9 0.00 0.33 0.00

12 0.00 0.00 0.33

15 0.33 0.33 0.33

18 0.67 0.33 0.00

21 0.33 0.33 0.67

24 0.33 0.00 0.00

27 0.00 0.33 0.33

30 0.00 0.00 0.00

Average rate

of diffusion

(mm/min.)

0.27 0.23 0.20

15

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

3 6 9 12 15 18 21 24 27 30

Par

tial

rat

e o

f D

iffu

sio

n (

mm

/min

.)

Time elapsed (min.)

Potassiumpermanganate

Potassiumdichromate

MethyleneBlue

Figure 6. A line graph showing the effect of time on the partial rate of diffusion of

potassium permanganate, potassium dichromate and methylene blue.

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larger particles need more energy to move than the smaller one (Chang, 1998). Thus, the

experiment supported the formulated hypothesis.

SUMMARY AND CONCLUSION

The effect of molecular weight on the rate of diffusion was determined using two

tests; glass tube test and agar-water gel test.

In the glass tube test, two moisten cotton ball with different substances,

hydrochloric acid (HCl) and ammonium hydroxide (NH4OH), was introduced to each

ending of the glass tube. A smoke ring was produced inside the glass tube. The position of

the smoke ring was closer to moisten cotton ball with HCl. The smoke ring is actually

NH4Cl, the product of the reaction of HCl and NH4OH. The closer distance of the marked

position of the smoke ring and the cotton ball with HCl indicates that the initial reaction of

HCl and NH4OH occurred closer to moisten cotton ball with HCl. It shows that NH4OH,

the substance with the lighter molecular weight, was able to diffuse faster.

In the agar-water gel, a drop of potassium permanganate (158 g/mole), potassium

dichromate (294 g/mole) and methylene blue (374 g/mole) were placed carefully and

simultaneously on individual wells in the agar-water gel. The diameter of the solutions

were measured and recorded, every three minutes, from 0 minute to 30 minutes.

Results showed that potassium permanganate produced the widest diameter (12

mm) after 30 minutes compared to potassium dichromate which produced a diameter

measuring 11 mm and methylene blue which yielded 10 mm diameter. It indicates that

potassium permanganate, with the smallest molecular weight, diffused the fastest (average

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rate of diffusion = 0.27 mm/min). Potassium dichromate yielded an average rate of

diffusion of 0.23 mm/min, and methylene blue with the largest molecular weight produced

the smallest diameter (10 mm) after the given time and had the slowest rate of diffusion

(0.20 mm/min).

Therefore, molecular weight and rate of diffusion of a substance have an inverse

relationship; the larger molecular weight, the slower rate of diffusion. Molecular weight is

only one of the factors that affects the rate of diffusion. It is recommended to further study

the other factors that affects the diffusion rate such as the concentration of the solution.

LITERATURE CITED

Campbell, Neil A. 1987. Biology. USA: The Benjamin/Cummings Publishing Company,

Inc. p. 164.

Chang, Raymond. 1998. Chemistry 6th ed. Boston: James M.

Duka, I.M., Diaz, M. G., and Villa, N. O. 2009. Biology I Laboratory Manual: An

Investigative Appproach 9th ed. p 34.

Mehrer, Helmut. 2007. Diffusion in Solids: Fundamentals Methods, Materials, Diffusion-

controlled Processes. Germany: Springer-Verlag Berlin Heidelberg. p. 1.

Rastogi, V. B. 1997. Modern Biology. New Delhi, India: Pitambar Publishing Company

Ltd. p. III-36.

Rowland, Martin. 1992. Biology. China: Thomas Nelson and Sons Ltd. p 31.