Chemistry, Third edition

11 · SOLUTIONS AND SOLUBILITY

whereas others will keep out only larger molecules (such as metal-complex ions or

sugars). The exact way that semipermeable membranes work is still poorly under-

stood. A common semipermeable membrane is cellulose acetate, which will not

allow sugar molecules or hydrated ions (made bulky by being attached to water

molecules) to pass through.

More about osmotic pressure

Osmotic pressure will be greatest at the start of an osmosis experiment, when the

concentration difference across the membrane is greatest. We should use the phrase

‘initial osmotic pressure’, but since the initial osmotic pressure is the only one in

which we are interested we shall drop the prefix ‘initial’.

Figure 11.11(a) illustrates a simple laboratory osmosis experiment. At the end of

the osmosis process the solution inside the thistle funnel has stopped rising.

We cannot accurately use the height the sugar solution has risen during the

osmosis as a measure of osmotic pressure. This is because the solution in the thistle

funnel is less concentrated at the end of the experiment than at the beginning. Since

a less concentrated solution is a less dense one, a simple measurement of hoveresti-

mates the osmotic pressure.

A better way of measuring osmotic pressure is to apply a known amount of exter-

nal pressure until the osmosis just stops. The applied pressure then exactly equals the

osmotic pressure of the solution. The principleof this type of measurement is shown

in Fig. 11.11(b). We start by connecting the top of the thistle funnel to a high pres-

sure air supply. The pressure of the air supply is slowly increased so as to just stop the

osmosis – this air pressure is equal to the osmotic pressure across the membrane.

Calculating osmotic pressure

The osmotic pressure may be calculated using the equation

cRT

188

Air pressure

just stops

sugar solution

from rising

Pressure

meter

(b) Air

2.0 mol dm–3

sugar solution

(a)

Semipermeable

membrane

0.1 mol dm–3

sugar solution

Solution rises

to this point

h

Fig. 11.11An example of osmosis: (a) the experiment; water enters the thistle funnel from the

less concentrated sugar solution (b) measuring osmotic pressure using an air supply.