chem

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.

Ex p e ri m e n t C olliga t i v e P rope r t ie s & O smoti c P res su r e

48 © H a nds-On L a bs, I n c . ww w . L a b P a q . c o m

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. B. How do these two freezing points compare?

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

52 © H a nds-On L a bs, I n c . ww w . L a b P a q . c o m

Questions - Part 2

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

B. 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?

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:

Π = MRT

Where: Π = 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?