Page:Text-book of Electrochemistry.djvu/57

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��VAPOUR PRESSURE OF SOLUTIONS. chap.

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���thing in a simpler way, and this development may fitly be introduced here.

A basin, Sy containing a liquid, is placed under a glass globe, A, from which the air can be pumped out (Fig. 12). The wide end of a funnel which is closed by a semi-permeable membrane, M, dips into the liquid in the basin ; the funnel

is provided with a long stem, i\ The funnel contains a solution, the solvent being the same as the liquid in 8y and the dissolved substance being non- volatile.

The liquid in 8 passes through the semi-permeable membrane and rises in r, until there is a hydro- static pressure on if, equal to the osmotic pressui-e of the solution Z.

In this case we have two semi- permeable media, namely, the mem- brane M and the vacuous space between the surfaces of the liquids in r and S, Suppose that the so- lution L contains N molecules of Pio. 12. molecule of dissolved substance, N

being large — that is, the solution a dilute one.

The osmotic pressure, and from this the height of the column of liquid in the tube r, can be calculated. The equation 'pv = BT gives us ^, when v and T are known. V is the volume which contains 1 gram-molecule of dis- solved substance. In this volume there are, according to the above assumption, N gram-molecules of solvent of molecular weight M — that is, -Mf grams of solvent, the specific gravity of which may be s. Therefore —

MN

���and consequently-

��V =

��i^ =

��ET xs MN

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