Ask Mendel's Mother Avogadro

B My son wants you to determine the molecular weights of several unknown substances. He has given you a semi-permeable sausage skin, two substances with known molecular weights and three unknown substances.

You must use the phenomenon of osmosis to find out these unknown molecular weights.


Materials and Methods

Making a solution

You must make up solutions of both types of substances. You do this by dissolving a known amount of the solute (the substance being dissolved), in a known amount of the solvent (the water in this case). From these two numbers (grams of solute and milliliters of solvent) you should first work out the concentration of the solution you have just made.

Concentration(gms/ml) = Number of grams of solute/mls of solvent


The Osmometer

The equipment use for measuring the osmotic pressure of a solution is very simple.


A solution of one of one of the known substances (molecular weight known) is placed into a thistle funnel (so called because it looks like a Scottish thistle), then a small piece of sausage skin is stretched over the mouth of the funnel and tied off around the edges, making sure that none of the solution can leak out.

The thistle funnel, solution and sausage skin is then inverted into a beaker. Into the beaker you pour a solution made up of one of the unknown substances. This fills the beaker and surrounds the inverted end of the thistle funnel.


You now watch to see if the liquid in the funnel moves up, down or stays the same.

Record your results.


Avagadro and Molar Solutions

My son has just read about the findings of Professor Avogadro. He has discovered that, if you weigh out exactly the molecular weight of a substance in grams, and then dissolve this in 1,000 ml (1 Liter) of water, you have a Molar solution.

For example, if the molecular weight of carbon dioxide is 48, and you weigh out 48 grams of carbon dioxide and dissolve that in 1,000 ml of water, you have 1 molar solution!

All molar solutions, of all substances, contain the same number of molecules of the substance. This is called the Avagadro number.

This is Avogardo's number :


So, a 1 molar solution of carbon dioxide in water, contains the same number of molecules as a 1 molar solution of a sugar such as glucose (which has a molecular weight of 180), even though there are more grams of the sugar dissolved!

Solutions of any substance, therefore, can be standardized. The molecular weight of a substance, measured in grams, is called one gram molecule or one mole of that substance (for example 48 grams of carbon dioxide would be "one gram molecule" or "one mole" of carbon dioxide).

If you dissolve "one mole" of a substance in 1,000 ml of water, you will have made a "1 molar" solution. Or, if you dissolved 1/10th of a mole in 1,000 ml of water you will have made a "0.1 molar" solution.

Solutions of all substances can therefore be standardized by always working with "molarity", rather than "grams/ml". The molarity of any solution of any substance can therefore be calculated using this formula:

molarity = (grams of substance / molecular weight) x (1,000 / mls of water used)

For example: 12 grams of carbon dioxide (molecular weight = 48) dissolved in 200 ml of water.

The molarity would be (12/48) x (1000/200) which would become (0.25) x (5) = 1.25 molar.


Osmotic Pressure Differences

If two different solutions are placed on opposite sides of a semi-permeable membrane, water will move through the membrane. Water will always move from the solution which has the most water (most dilute) and into the solution which has the least water (most concentrated).


The practical effects of this is that the level of the solution in the osmometer (inside the thistle funnel), will move up or down, depending on the difference in concentration.




When the solutions on both sides of the membrane have the same molarity, then the osmotic pressure of both solutions is the same, and there will be no net movement of water into or out of the thistle funnel, and the liquid level will remain constant.

This is the point of equilibrium.

When you find the point at which the liquid does not move in an osmometer (the equilibrium point), then you know that the molarity of the solutions on both sides of the membrane are the same!


The Calculation

Once you have reached the equilibrium point, you are now in a position to calculate the molecular weight of the unknown substance.


M1 = molecular weight of the known substance,
M2 = molecular weight of the unknown substance,

V1 = volume of solution of known substance,
V2 = volume of solution of unknown substance,

G1 = number of grams of known substance,
G2 = number of grams of unknown substance.

then, at equilibrium:

molarity of known solution = molarity of unknown solution

(G1/M1) x (1000/V1) = (G2/M2) x (1000/V2)

when this formula is rearranged, it becomes -

M2 = (G2/G1) x (V1/V2) x M1


Unknown molecular weight = (G2/G1) x (V1/V2) x M1

B Now it is your turn! Use the simulation of Osmosis to find the missing molecular weights.

Start Here Use the Osmometer
begin using the osmometer to find the missing molecular weights.

Science at a Distance
© 1999 Professor John Blamire