Ramsay-Young rule

Another useful approximation is the Ramsay-Young rule, which states that for two related substances, the ratio of the temperatures at which they exert the same vapor pressure is constant.7 That is,... 

The extent to which the Ramsay-Young rule, represented by equation (27.26), is applicable may be illustrated by reference to water and ethanol. The vapor pressure of water is 12.2 mm. of mercury at 287.5 K, and ethanol has the same vapor pressure at 273.2 K the ratio of these temperatures is 287.5/273.2, i.e., 1,052. Since the normal boiling points are 373.2 E for water and 351.5 K for ethanol, these are the temperatures at which both have the same vapor pressure of 1 atm. the ratio of these temperatures is 373.2/351.5, i.e., 1.062. The two ratios thus agree to within about one per cent. 

It is of interest to note that equation (27.28) is essentially equivalent to the Ramsay-Young rule. In the first place, it is based on equation (27.16), and in the second place, it supposes that the constant C has the same value for all liquids if this were the case, Ca Cb in equation (27.24) and hence Ci in equation (27.25) would have to be zero. Hence, (27.28) would lead directly to the Ramsay-Young equation. 

Show that if Tx and Tb are the temperatures at which two liquids have the same vapor pressure, then by the Ramsay-Young rule, log Tx = log Tb + const. the plot of log Tx against log Tb should thus be linear. 

By means of these data and those given for carbon tetrachloride in Exercise 1, determine how closely the Ramsay-Young rule is obeyed. (Plot the vapor pressure as a function of temperature in each case, and determine the temperatures at which the two liquids have the vapor pressures 120, 240 and 360 mm., and then plot log Ta against log Tb-)... 

Ramsay and Young (1886) found that in the vicinity of the boiling point the quantity T(dp/dT) is approximately constant for many liquids. Show that this is a consequence of the Ramsay-Young rule, and that it leads to the relationship AT cTAP, where AT is the increase of boiling point for an increase AP in the external pressure for liquids obeying Trouton s rule, c should be approximately constant and equal to 1.2 X 16 if AP is in mm. (Craft s rule). Estimate... 

Provided the vapor pressures of a reference liquid are known over a range of temperatures, it is possible, by means of the Ramsay-Young or DUhring rules, to establish the complete vapor pressure-temperature variation of another substance from one datum for the latter. It should be noted that the results obtained cannot be very precise, for the equations (27.26) and (27.27) can be exact only if the vapor pressures are represented by the linear equations (27.15), and if the constant Cl in equation (27.25) is zero. ... 

A. Findlay (1902) found that Ramsay and Young s rule for the vapour pressures of pure liquids ( 89) has an analogue in the case of solutions. If TA, TA1 are two temperatures at which the substance A has the solubilities s, s, and TB, TB two temperatures at which another substance has the same solubilities in the given solvent then ... 

An extension of Ramsay and Young s vapour pressure rule ( 8.VIIIK) to the temperatures at which two substances have equal solubilities, Findlay,... 

Ramsay and Young also found that for a given pressure the ratio of the absolute temperature to the absolute critical temperature is approximately consent. This is an extension of Guldberg s rule ( 7.VII C). Adams found that the b.p. abs. at p atm. is related to that at 1 atm. by the formula ... 

Porter showed that Ramsay and Young s rule follows from the equation ... 

The results of experiment are in complete harmony with the requirements of the Phase Rule, as is shown, for example, by the determinations of the vapour pressure of water carried out by Magnus, Regnault, Battelli, Ramsay and Young, Juhlin, Thiesen and Scheel, Holborn and Henning, Holborn and Baumann, Scheel and Heuse, and others. 

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