Determining the Relationship Between Diffusion Rate and Solution Concentration

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Experimental Lab 5: Diffusion and Osmosis

Introduction

In order for cells to maintain homeostasis, they must allow substances to move in and out of the cell. There are four different ways in which substances move across differentially permeable biological membranes, including simple diffusion, facilitated diffusion, active transport, and vesicular transport. The purpose of experiment five is to practice making percent solutions as well as understanding the principles of simple diffusion and osmosis. Exercise one uses the reaction of sodium chloride with silver nitrate to produce sodium nitrate and silver chloride. Exercise two measures the rate of osmosis with different percent solutions of sucrose as a substance. I predict that the higher percent solutions of NaCl will produce a precipitate that moves faster and further through the agar mediums. I also hypothesize that the rate of osmosis will increase as the percent solutions of sucrose increase, causing a larger difference in weight of the bag.

Materials and Methods

The detailed procedure and list of materials can be found in pages 56-60 of the Biology 203L Lab Manual. In exercise one, we made 2 mL of NaCl solutions instead of 5 mL. In exercise two, we stopped the experiment at the 45-minute time point instead of at 60 minutes. Also, my lab partner and I only did two of the five assigned percent solutions, the control and number five.

Experimental Data

Table 5.1 Calculations of Percent Sodium Chloride Solutions

% NaCl solution Solid NaCl (g) Final Volume

1% 0.02 g 2 mL

20% 0.4 g 2 mL

Table 5.3 Percent Sucrose Solutions to Make

Percent solution to make Final volume needed (mL) Volume of 60% stock needed (mL) Volume water needed (mL)

20% 15 mL 5.0 mL 10.0 mL

40% 15 mL 10.0 mL 5.0 mL

Calculated Results

Table 5.2 Diffusion Data

Time (min) Distance of AgCl band movement (mm)

1% NaCl 20% NaCl

1 0.8 mm 2.0 mm

5 1.2 mm 2.5 mm

30 2.2 mm 18 mm

45 3.2 mm 19 mm

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60 4.0 mm 20 mm

Rate of Cl- Diffusion

1% NaCl Solution: 4.0 mm/hr

20% NaCl Solution: 20.0 mm/hr

Table 5.4 Tubing Number, Weight, and Change in Weight

Time (min) 1 2 3 4 5

weight ∆wt weight ∆wt weight ∆wt weight ∆wt weight ∆wt

0 15.2g 0.0g 15.4g 0.0g 16.5g 0.0g 18.8g 0.0g 15.4g 0.0g

15 15.1g -0.1g 16.1g 0.7g 18.3g 1.8g 20.7g 0.9g 13.9g -1.5g

30 15.0g -0.2g 17.5g 2.1g 19.3g 2.8g 22.2g 3.4g 11.5g -3.9g

45 14.6g -0.6g 17.9g 2.5g 20.0g 3.5g 24.6g 5.8g 9.4g -6.0g

Rate of osmosis

1: -0.6g /45 min

2: 2.5g /45 min

3: 3.5g / 45 min

4: 5.8g / 45 min

5: -6.0g / 45 min

Discussion

My hypotheses that the rates of diffusion have a direct relationship with concentration of substance were correct. Both graphs show that as the concentration of NaCl or sucrose increased, the rate of diffusion/osmosis also increased.

The data in table 5.2 presents a direct relationship between chloride ion concentration and rate of diffusion in agar. Simply, as the percent NaCl solution increased, the precipitate formed moved faster through the gel medium. The graph corresponding to table 5.2 measures the rate of chloride ion diffusion. The reactants NaCl (solid) and AgNO3 (agar) combine to form a solid precipitate of AgCl and aqueous NaNO3. The chloride ion diffusion is measured instead of the sodium ion diffusion because silver chloride (AgCl) is the precipitate that contains the chloride ions that moved down the tube while sodium nitrate (NaNO3) is the aqueous liquid that contains the sodium ions. Therefore, we measured the movement of chloride ions, not sodium ions. If the agar concentration was increased to 10% instead of 2%, the rate of chloride ion diffusion would be much lower, as it would take longer for the precipitate to pass through the agar.

The data in table 5.4 and the corresponding graph prove that concentration of sucrose and rate of osmosis have a direct relationship. We know from the procedure on page 60 of the Biology 203L Lab Manual that tube #1 has tap water in the bag and tap water in the beaker, #2 has 20% sucrose in the bag and tap water in the beaker, #3 has 40% sucrose in the bag and tap water in the beaker, #4 has 60% sucrose in the bag and tap water in the beaker, and lastly #5 has tap water in the bag and 60% sucrose in the beaker. The concentration of sucrose increases in each tube, as does the rate of osmosis. Tubes 4 and 5 both measure the rate of osmosis between 60% sucrose and tap water. However, the rate of osmosis in g/45 min is larger in treatment 5 than treatment 4 because in treatment 4 there is only 15 mL of 60% sucrose (in the bag) while there is 125 mL of 60% sucrose in treatment 5 (in beaker). Treatment 5 would predictably have a higher rate of osmosis because there is more mL of sucrose that water would diffuse with.

Sources of error for this experiment were slim, most of them including equipment error. For example, it was difficult to weigh out 0.02g of NaCl in exercise one, as the scales provided were only accurate to 0.1g. In addition, using the rulers provided were not ideal for measuring the distance the precipitate traveled at each time interval. Similarly in exercise two, there may have been excess weight from the substance (water or sucrose) in the beaker still on the bag that was weighed at each time interval. It was virtually impossible to blot off all of the excess water (in tube 5, sucrose) from the outside of the bag, allowing a marginal error of additional weight to be measured. Despite these possible errors, the data collected shows that there was little presence of a third variable affecting the expected relationship between the observed variables.

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