PASCO ME-6940 Diffusion-Osmosis Apparatus - Manual (Page 10)

M o d e l N o . M E - 6 9 4 0 S a m p l e D a t a
10
Table 1: Fluid Volume
Table 2: Pressure
Table 3: Concentration
Analysis Questions and Answers
1. Is the change in fluid volume equivalent on each side of the apparatus? In other words, is the loss of volume on
one side the same as the gain of volume on the other side?
Answer: Yes, the change in volume in the two columns is equivalent. 10 mL of water moved from column #1 to col-
umn #2, resulting in a 10 mL increase in volume in column #2.
2. Is the change in pressure equivalent on each side of the apparatus? In other words, is the loss of pressure on one
side the same as the gain of pressure on the other side?
Answer: No, the change in pressure in the two columns is not equivalent. The increase in pressure in column #2 was
greater than the decrease in pressure in column #1. Boyle's law states that at constant temperature, the absolute pres-
sure and the volume of a gas are inversely proportional. So, as water moves across the membrane from column #1 to
column #2, the volume of available space within column #1 increases. According to Boyle's law, this results in a
decrease in pressure within the column. But, as the water moves into column #2, the volume of free space decreases,
and the pressure increases. So, why is the pressure change not equivalent? Let's look at the combined gas law equa-
tion PV = nrT. The equation to find pressure is P = nrT / V. So, the greater the volume in the denominator, the smaller
the pressure, and vice versa. For our experiment, this means that as more and fluid enters column #2 (aka - decreased
volume of free space in column #2), the pressure will continue to increase. In column #1, the volume of free space is
increasing as water moves to column #1, resulting in a decrease in the pressure. One other factor affecting the
non-equivalent change in pressure is the molarity of the solutions (n). As water moves into column #2, the solution
becomes more and more dilute, resulting in a decrease in n. Column #1 only contains water, so n remains constant.
When n and V both decrease in column #2, a compounded increase in pressure results in column #2. N remains con-
stant in column #1, so only the change in volume affects the pressure of this side. Therefore, the change in pressure is
not equivalent.
Solution Initial Volume (mL) Final Volume (mL) Change in Volume
Absolute Pressure 1 Distilled water 40 30 -10
Absolute Pressure 2 2.0 M Sucrose 40 50 +10
Solution Initial Pressure
(kPa) Final Pressure
(kPa) Change in
Pressure Slope (kPa/hour)
Absolute
Pressure 1 Distilled water 102.17 89.03 -13.14 -0.613
Absolute
Pressure 2 2.0 M Sucrose
solution 102.49 126.27 +23.78 +0.965
1-2 Differential
Pressure (Abs. pressure 1 -
Abs. pressure 2) -0.32 -37.24 -37.24 -1.580
Solution Initial Concentration (mol/L) Final Concentration (mol/L
Absolute
Pressure 1 Distilled water 0.0 0.0
Absolute
Pressure 2 2.0 M Sucrose
solution 2.0 1.6