44 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



(b) Chemical Methods. 



When the substance is an acid or a base the molecular weight can 

 be determined by chemical methods, 

 (i) In the case of an acid : 



The molecular weight can be calculated from the amount of standard 

 alkali required to neutralise, using phenolphthalein as indicator, a known weight 

 of the acid, according to the equation 



H acid + NaOH = H 2 O + Na acid. 

 e.g. x c.c. *iN NaOH = y gm. of acid. 



.-. 40 gm. NaOH = 4 x ? 



x x 0-004 

 ass mol. wt. of acid. 



If the acid be dibasic or tribasic, two or three molecules of sodium hy- 

 droxide will be required. The presence of such an acid will be indicated by 

 titrating the acid using methyl orange, or alizarin red, and phenolphthalein as 

 indicators. The acid salt will be neutral to methyl orange or alizarin red, the 

 neutral salt to phenolphthalein. The basicity of the acid is definitely ascer- 

 tained by the analysis of the salt and the free acid. 



It is most usual to employ the silver salt of an acid. A quantity 

 of the salt is prepared by adding silver nitrate to the neutral solu- 

 tion of the acid, filtering off the silver salt, washing and drying it. 

 A known weight is heated in a crucible and the metallic silver obtained 

 is weighed. 



a gm. of silver salt gave b gm. of silver. 

 If the acid be monobasic it will contain i atom of silver, 



107-9 gin- of silver will be contained in IO 7 9 * a gm> Qf s j lver salt> 





 Since the silver replaces i atom of hydrogen 



.-. I0 7'9 x a _ IO j. Q + r is the mol. wt. of the acid. 



o 

 If the acid be dibasic it will contain 2 atoms of silver, 



107*9 x 2 gm. of silver will be contained in I0 7 9 x 2 x a ^ Qf s ij ver ga j tf 





 Since the silver replaces 2 atoms of hydrogen 



.. i 107 ' 9 x 2 x _ ( I07 - 9 x 2) + 2 is the mol. wt. of the acid. 

 



The zinc salt or barium salt is also sometimes employed ; a known 

 weight of salt is heated with a drop of concentrated sulphuric acid in 

 a crucible ; zinc or barium sulphate is obtained from which the amount 

 of barium or zinc is calculated. 



