Determination of Molecular Mass of Acids by Silver Salt Method
This method is based on the fact that they form insoluble silver salts, which upon heating decompose to leave a residue of metallic silver.
Procedure:
The unknown acid is dissolved in water and treated with a slight excess of ammonium hydroxide. The excess of ammonia is boiled off. To this sufficient quantity of silver nitrate is added when a white precipitate of silver salt is obtained. The precipitate is separated by filtration, washed successively with water, alcohol and ether and dried in the steam oven.
About 0.2 g of the dry silver salt is weighed into a crucible and ignited until the decomposition is complete. Ignition is repeated until the crucible with the residue of silver has attained constant weight. The molecular mass of the acid is then calculated from the mass of the silver salt taken and the mass of the residue of metallic silver obtained from it.
Calculations:
Weight of unknown carboxylic acid = W g
Weight of silver salt of that acid = X1 g
Weight of metallic silver = X2 g
i.e. X2 g of silver are obtained from X1 g of silver salt
Then 108 g of silver are obtained from = (X1/X2) X 108 g of silver salt
Thus, molar mass of silver salt of carboxylic acid = (X1/X2) X 108 g
For monocarboxylic acid, molar mass = (molar mass of Ag-salt) - (atomic mass of Ag) + (Atomic mass of H)
= (108 X1/X2) − 108 + 1
= (108 X1/X2) − 107
For polybasic carboxylic acid of basicity (n), molar mass = n(Molar mass of Ag-salt) - n(atomic mass of Ag) + n(atomic mass of H)
= (n108 X1/X2) − 108n + 1n
= (n108 X1/X2) − 107n
About 0.759 g of silver salt of a dibasic acid was ignited when a residue of 0.463 g of metallic silver was left. Calculate the molecular mass of the acid.
Answer
Solution: 0.463 g of metallic silver is left by silver salt = 0.759 g
216 g of metallic silver is left by silver salt = ( 0.759/0.463) X 216 = 354 g
so, molecular mass of dibasic acid = (mol. wt of Ag salt − 2) X 108 + 2 = 140 g
For the calculation of molecular weight of dibasic acid using silver salt method, a straight line as shown is obtained when a graph is plotted between weight of silver salt and weight of silver. Find the molecular weight of acid (Take √3 = 1.7)
Answer
Solution: From the graph-
Wt.of silver salt / Wt.of silver = tan60°
tan60° = √3 = 1.7
for dibasic acid, basicity = 2
Now, Molar Mass of Acid = [(108 x 1.7) − 107] × 2
or, Molar Mass of Acid = (183.6 − 107) × 2
or, Molar Mass of Acid = 76.6 × 2 = 153.2
or, Molar Mass of Acid = 153.2gm
