Question

Colligative Properties & Osmotic Pressure

Peter Jeschofnig, Ph.D. Version 42-0149-00-01

Lab Report Assistant                                                                                            

This document is not meant to be a substitute for a formal laboratory report. The Lab Report Assistant is simply a summary of the experiment�s questions, diagrams if needed, and data tables that should be addressed in a formal lab report. The intent is to facilitate student�s writing of lab reports by providing this information in an editable file which can be sent to an instructor

Observations & Questions for Part 1                                                              

Record your observations and your time and temperature data in tables. Use one table for the pure water and one table for the salt solution.                                                                                        

Data Table 1: Pure Water and Salt Solution
Seconds	Distilled H20 Room temp	Distilled H20 Ice bath	Saltwater Room temp	Saltwater Ice bath
0	23	23	17	23
30	23	10	20	7
60	23	4	22	1
90	22.5	1	22	0
120	22.5	0	22	0
150		0	22	-1
180		-1	22	-1
210		-1	22	-2
240		-2		-2
270		-1		-2
300		-1		-2
330		-1		-2
360		-1
390		0
420		0
450		0
480		0
510		0
540
570
600
630
660

Make two graphs of your data. On one graph plot the data from the pure water. On the other graph plot the data from the salt solution. On both plot temperature on the y-axis and time on the x-axis.

A. Record the freezing point of the pure water and the freezing point of the salt solution.

Freezing point of salt Sol: -2 degree Celsius

Freezing point of pure H2O: 0 degree Celsius

B. How do these two freezing points compare?

The freezing point of salt water is lower than the freezing

point of fresh water. Salt in the water lowers the freezing point

of water.

C. What are some practical applications of freezing point depression, boiling point elevation, and vapor pressure lowering?

                                                            Some practical applications of freezing point depression

                                        are antifreeze in a radiator and salt on the road used to melt

                                        ice in the winter. Some practical applications of boiling point

                                        elevation are a sealed container and possibly a pressure cooker.

                                        Some practical applications of vapor pressure lowering are freeze

                                        drying and steam engines.

Questions - Part 2                                                                                                  

To what biological structure is the dialysis bag comparable? How is it similar? How is it different?

The plasma membrane is comparable to the dialysis bag.

The similarity between them is that they are both semipermeable.

The difference is the dialysis bag acts as a permeable membrane,

                                        however, purely based on its pore size, the small molecules are

                                        able to get through but the bigger molecules are unable to. The

                                        plasma membrane is a more complex system that uses both active

                                        and passive transport to allow the molecule to move through.

                                            

In biological systems if a cell is placed into a salt solution in which the salt concentration in the solution is lower than in the cell, the solution is said to be hypotonic. Water will move from the solution into the cell, causing lysis of the cell. In other words, the cell will expand to the point where it bursts. On the other hand, if a cell is placed into a salt solution in which the salt concentration in the solution is higher than in the cell, the solution is said to be hypertonic. In this case, water will move from the cell into the solution, causing cellular death through crenation or cellular shrinkage. In your experiment is the Karo� hypertonic or hypotonic to the egg?

The Karo is hypertonic to the egg, because during the 24

hours, the egg started to float on top of the Karo syrup

and there was water loss.

C. Historically certain colligative properties � freezing point depression, boiling point elevation, and osmotic pressure � have been used to determine molecular mass. (Now there are instrumental methods to determine this.) Of these three, osmotic pressure is the most sensitive and gives the best results. Molecular mass can be found according the following equation:

? = MRT

Where:                  ? = osmotic pressure,

M = molarity of solution,

R = the ideal gas constant (0.0821 L�atm/mol�K), and

T = Kelvin temperature.

Sample Problem

0.125 grams of a starch is dissolved in 100 mL of water at 25oC and has an osmotic pressure of

5.15 mmHg. What is its molecular mass?

Since the gas constant, R, requires atmospheres as pressure units, we have to convert 5.15 mmHg to atmospheres:

5.15 mm Hg * 1 atm/760 mm Hg = 0.00678 atm.

0.00678 atm = M (0.0821) (273 + 25 = 298 K); We solve for M = 0.00678/(0.0821*298);

M (molarity) = 2.77 x 10 �4

Molarity = moles/L: 2.77 x 10�4 = moles/.1 = 2.77 x 10-5 moles of starch; If 2.77 x 10-5 moles = 0.125 grams,

Then 1 mole = 0.125 g/2.77 x 10-5 = 4512 g/mole = molar mass of starch.

Problem for Lab Report:

At 23.6�C, 0.500 L of a solution containing 0.302 grams of an antibiotic has an osmotic pressure of 8.34 mmHg. What is its molecular mass?

8.34 mmHg(1atm/760 mmHg)

=0.010974 atm

0.010974 atm/ (0.0821L.atm/K.mol)(296.6K)

=0.000451 mole/L

0.500L(0.000451 mole/L)

=0.000225

=0.302 gr/0.000225

=1342.22g/mol

                             

                                        The molecular mass is 1342.22 g/mol.

So after I completed this lab, I was asked the following question:

What changes would you make, if any, if you did this lab again? Include a discussion of differences between the individual data sets

Answer 1

1. About freezing point of salt solution and distilled water I could have explained why we chose the values as above because if it is pure solvent like water then it is able to freeze early i.e at somewhat higher temperature but if it is a salt solution then that salt is responsible for freezing point depression of the solution.

2. "Depression in freezing point is used to prevent car fuel from freezing by adding anti-freeze." The antifreeze also causes an elevation in the boiling point, so it helped with a car overheating in the summer. In the USA, there was an antifreeze called Prestone that advertised itself as "winder-summer protection."

Their most prominent use is to find molar mass of an unknown compound dissolved in a solvent. Depression in freezing point is used to prevent car fuel from freezing by adding anti-freeze. Relative Lowering of vapour pressure in case of solution of two liquids is used in their distillation. Osmosis finds an interesting use in desalination of water through a process called reverse osmosis. It provides drinking water from sea water.

3. The plasma membrane is comparable to the dialysis bag. Because They are both semipermeable allowing certain things to pass though membrane and other not to pass through membrane.