Show OSMOTIC PRESSURE analogy between gasses and dilute solutions method of making test written for the mining review if an aqueous solution of a substance is contained in a vessel whose walls at are e permeable to the molecules of water but not permeable to the molecules of the dissolved substance and the vessel is immersed m in water the latter will enter the vessel and the pressure on the walls will increase until equilibrium results after which no more water will enter this pressure is called osmotic pressure assuming that osmotic pressure is due to the impact of the molecules of the dissolved substance then the number of impacts in unit time must be proportional to the number of molecules in a unit volume this is the molecular conception of gaseous pressure since in iii the case of a gas volume is inversely tion to pressure boyles law the same relation would hold good in dilute aqueous solutions it has been also found that osmotic pressure of a specified mass of a substance in dilute solution is the same as the pressure exerted by the same mass of the substance existing as a gas at the same temperature this is equivalent to saying that charles law also holds good for dilute aqueous solutions inasmuch as concentration cent ration of solution corresponds with gaseous solutions osmotic pressure always increases with a rise in temperature which is the same thing that occurs in the case of a gas this illustrates the of gay Lus sacs law to dilute solutions in the same way law may be applied to substances in dilute solutions this extension is stated in van colfs law of osmotic pressure equal volumes of different solutions at the same temperature and osmotic pressure contain equal numbers of molecules cu les which are the same as would be contained in equal volumes of gases at I 1 the same temperature and pressure to apply this law it Is necessary to find the mass of a substance present in a solution which is of equal osmotic pressure with another solution containing a known molecular weight both substances then contain equal numbers of molecules of the dissolved substances and as the molecular weight of one substance is known the molecular weight of the other can be found there are many apparent exceptions to these laws applying equally to gases and to dilute solutions this discrepancy is also found in gases as shown by the following case the pressure of the vapor obtained by heating chloride is greater than the pressure from avo badros law this is explained by the assumption that the vapor consists of molecules of chloride but as the observed pressure only agrees with the calculated pressure when it is assumed that the vapor consists of equal numbers of molecules of amonia and chlorine so the apparent abnormal osmotic pressure of many solutions are made to agree by assuming that these solutions are more or less dissociated measurement of osmotic pressure A membrane of copper is precipitated within the walls of a porous clay cup in the following manner the cup is filled with a weak solution of potassium sulphate and immersed in a vessel containing the same solution until the outer level is near the margin of the cup then a curren from a dynamo is passed between a cylindrical copper electrode surrounding the cup and a platinum electrode within it As the liquid rises in the cup it is removed and in a short time the air is all removed from the porous clay then the cup is filled with a solution of potassium ferro cyanide and immersed in a solution of copper sulphate the current is then turned on again and the potassium ferro cyanide ions are driven into the clay from the inside of the cup while the copper ions are forced in from the outside the resistance of the cup gradually rises as the membrane is formed being from 1500 to ohms at the time when the membrane is considered as complete when the membrane is formed the cup is then filled with the solution to be tested closed with a copper bearing a mercury thermometer and immersed in water the osmotic pressure rises for a number of hours the rise being indicated by the rise of the mercury and when the membrane breaks the mercury descends the maximum reading of the mercury column ib taken as the osmotic pressure of the solution A L SWEETSER E M denver colo |