﻿of Phosphorus , Sulphur, and Aldehyde 531 



given moment is not simply proportional to the diminution 

 of the pressure. 



Suppose that at the commencement of the experiment, the 

 partial pressure of the aldehyde were =a millim., that of the 

 oxygen = b millim., and that of the nitrogen = N millim. 

 And let P be the total pressure of the gas at any time, t 

 minutes after the beginning of the experiment. Suppose also 

 that at the same instant x millim. of oxygen have united with 

 2x millim. of aldehyde. At first the acetic acid formed 

 remains as vapour, and as its density at 20° is double the 

 normal value, we shall have 2 volumes of aldehyde-vapour 

 and 1 volume of oxygen condensing to form 1 volume of 

 acetic-acid vapour. That is, the acetic-acid vapour will 

 occupy the same volume and exert the same pressure as the 

 oxygen from which it is formed ; and the pressure of the 

 mixture will be 



P = (a-2a) + (b-x) -M + N 



The pressure at the beginning of the experiment (P ) was 

 equal to a + b + N, so that we obtain 



2tf=P -P. 



After a time the acetic-acid vapour will reach its maximum 

 pressure (say m) , after which liquid acetic acid will be formed, 

 and its quantity will be proportional to (x — m), that is 

 = k(x — in). If we assume that the quantity of aldehyde 

 dissolved in the acetic acid follows Henry's law, it will be 

 proportional to the pressure of the gaseous aldehyde and to 

 the quantity of liquid acetic acid; that is, to kf (x — rri)p 2 , where 

 p 2 is the pressure of the aldehyde-vapour. 



The pressure of the aldehyde-vapour at any moment is its 

 original pressure diminished by the part which has undergone 

 chemical reaction and by the part dissolved in the acetic 

 acid; that is 



p 2 =z a — 2x — H(a — m)p a , 



or _ a — 2 x . . 



^-\ + h\x-m) W 



It may be pointed out that even if the assumption that 

 aldehyde-vapour dissolved in acetic acid follows Henry's law 

 is not strictly accurate, no great error will be committed, 

 because the quantity dissolved is not large. 



We also have the equation expressing the total pressure P 



