236 THE REALITIES OF MODERN SCIENCE 



volume is 1 liter the " concentration " of gas molecules 

 is 1 mole per liter. In general if the volume is F, the 

 concentration, represented by c, is l/V. In the equa- 

 tion PV = RT, if we substitute c for l/V we have 

 P = RTc, as the expression for the pressure of the dis- 

 solved molecules of the gas. 



This equation is not limited to the case we have dis- 

 cussed. No matter how the molecules are introduced 

 into the liquid we should expect this pressure. For 

 example, the molecular weight of cane sugar, which has 

 the formula C 12 H 22 On, is 12(12) + 22(1.008) + 11(16) 

 or 342.2. Ten grams of sugar in a liter of water is 

 10/342 of a mole per liter. The value of c is then 

 0.029. Suppose the solution is at 30 C. or approximately 

 300 absolute. If we calculate the pressure which the 

 molecules of the sugar would exert on each sq. cm. of 

 a membrane inserted in the solution, we find it to be 

 about 0.7 of an atmosphere or 10 pounds to the square 

 inch. 



In the case of a gas we can measure such a pressure 

 directly by measuring that of the gas above the solu- 

 tion, but it is neces- 

 sary, if we wish to check 

 these assumptions for 

 the sugar solution, that 

 we should measure the 



pressure in the solution. Imagine now a vessel like 

 that of Fig. 28 to be divided by a membrane which 

 separates solutions of the same character but of 

 different concentrations. There will be a pressure of 

 pi = RTci on the left of the membrane and of pz = RTc 2 

 on the right. Assume that the membrane is movable 



