56 IONISATION 



[OH~] =the fraction of the total water which is disso- 



ciated into hydroxyl ions 



and [H 2 O] =the fraction of the total water which is left 



undissociated. 



Stated in words, the product of the concentration of hydrogen 

 ions and hydroxyl ions is always equal to a constant quantity K w , 

 The value of K w , the ionisation constant of water, depends only 

 on the temperature. 



At = io,ooo,ooo,ooo,ooo7ooo ............... ' 



At 2S r K 





100,000,000,000,000 



28 



K = ioo,ooo,ooo,ooo,ooo ....... 



48 

 At 100 C. K,, - .................. (4) 



100,000,000,000,000 



To save writing those cumbrous fractions, the index notation 

 is used. Thus fraction 



(l)is : = 0-01xlO- u =C w at 0C. 



(2) is = 1 xlO- 14 =C w at 23 C. 



(3) is : 28 X10- 14 =C W at 40 C. 



(4) is : 48 X10- 14 =C W at 100 C. 



Now, as the quantity of water dissociated is so very small 

 compared with the total quantity of water, it is clearly legitimate 

 to put [H 2 O] =1 in the mass action equation, which now becomes : 



[H+] X[OH-] =*.. 



It is obvious that H+ and OH~ are produced in equal amounts, 

 and therefore [H+] =[OH~] =7^7- 



Between 22 and 23 C. water has a dissociation constant with 

 which it is convenient to work, and measurements of hydrogen 

 ion concentrations are usually made at this temperature or 

 referred to this temperature ; 



i.e. at23C., K W =IQ- U ; 

 /. [H+]x[OH-]=10- 14 . 

 [H+] is therefore equal to S /10- 14 =10- 7 , 

 and [OH-] s/lO^ 4 =10~ 7 . 



It is usual to write H* for H+ and OH' for OH~ 



