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 0C. K w 1^000,000,000,000*000 ............... ^ 



At 23 C. K w = 100j000j00 o 5000) ooO .................. (2) 



28 



^^100,000,000,000,000 .................. (a 



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



100,000,000,000,000 



To save writing those cumbrous fractions, the index notation 

 is used. Thus fraction 



(1) is = 0-01 xlO- 14 =C w at C. 



(2) is = 1 xlO~ 14 =C U at 23 C. 



(3) is = 28 XlO~ 14 =C W at 40 C. 



(4) is = 48 xlO- 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-] =K W . 



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

 and therefore [H+] =[OH-] WAV 



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. at 23 C., K w =10~ 14 ; 

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

 [H+] is therefore equal to 7lO~ 14 -10 ~\ 

 and [OH-] 



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



