Question

2. Calculate the gravimetric factor for the following: (a) Cr2O3 in Ag2CrO4 (b) CaO in CaCO3,

3.Describe briefly the mechanism of precipitate formation.

Answer 1

2. (a)

Mass of Ag₂CrO₄ = 331.73 g/mol

Mass of Cr₂O₃ = 151.99 g/mol

Gravimetric Factor = 331.73 / 151.99

Gravimetric Factor = 2.182

(b)

Mass of CaCO₃ = 100.089 g/mol

Mass of CaO = 56

Gravimetric Factor = 100.089 / 56

Gravimetric Factor = 1.787

3.

MECHANISM OF PRECIPITATION

The purity and the filterability of a precipitate greatly depend upon the particle sizeof the precipitate. The particle size is determined by the relative rates of the two processes namely, nucleation which is the production of extremely small particles(nuclei) capable of spontaneous growth and particle growth (crystal growth) whichis the growth of the nuclei.Once the nucleation process starts, it proceeds ultimately to form the precipitate.This can be represented with the help of actual particle size increase as given below:Ions (10-

8 cm) → Nucleation clusters (10-8 to 10-7 cm) → Colloidal particles (10-7 to 10-4 cm) → Precipitate (> 10-4 cm)

This shows that the nucleation clusters pass through the stage of colloidal particlesize prior to precipitation. Problems which arise with certain precipitates includecoagulation or flocculation of a colloidal dispersion of a finely divided solid to permit its filtration and to prevent its repeptisation upon washing the precipitate.Hence, the effect of colloidal state on the process of precipitation is also important.The effect of the rate of precipitation on the particle size has been studied by vonWeimarn (10). He found that faster the precipitation, smaller is the particle size.He also found that the rate of precipitation is dependent on the relative super-saturation.According to von Weimarn,Where, Q-S = relative supersaturation,Q = molar concentration of the mixed reagents before precipitation,S = molar solubility of the precipitate at equilibrium,K = a constant