CHEMICAL ANALYSIS 9 



Where the substance possesses molecules of water of 

 crystallization, the weight of these must be added to the 

 sum of the molecule proper, in order to arrive at the right 

 figure for the normal solution. Thus 



H 2 C 2 4 +2H 2 m.w. = i26 N/i=63 grm. per litre 

 (oxalic acid). 



H 3 C 6 H 5 7 +H 2 m.w.=2io N/i=70 grm. per litre 

 (citric acid). 



H 2 C 4 H 4 6 m.w. = i5o N/i= 75 grm. per litre 

 (tartaric acid). 



HC 2 H 3 2 m.w. =60 N /i =60 grm. per litre 

 (acetic acid). 



Some salts with enlarged molecules : 



Ferrous sulphate, FeS0 4 +7 H 2 molecular weight =278 

 Copper sulphate, CuSO d -|- 5 H 2 =249 



Lead acetate, Pb(C 2 H 3 2 ) 2 +3 H 2 ,, =379 



Sodium thiosulphate, Na 2 S 2 3+5. H 2 ,, ,, =248 



The hydrogen equivalent of some reagents is not so 

 easily arrived at ; for instance, potassium permanganate, 

 variously written as 



K 2 Mn 2 8 and KMn0 4 . 



The molecular weight of the first formula is 316, and of the 

 second is 158. Nevertheless, the normal solution is not 

 158 grammes per litre, but is 31-6 grammes per litre. 

 This is because potassium permanganate does not react 

 as its formula might suggest as a manganate, but reacts 

 as a double salt of potassium and manganese, as if it were 

 written thus 



K 2 + Mn 2 7 , 



and in reaction the latter oxide is reduced and oxj^gen 

 liberated ; thus 



K 2 Mn 2 8 +3 H 2 SO 4 =K 2 SO 4 +2 MnSO 4 +3 H 2 0+5 O 

 316 80 



We here see that 316 grammes of permanganate of potash 

 liberate 80 grammes of oxygen, which are chemically 

 equivalent to 10 grammes of hydrogen, and so 31-6 



