Question

The percentage of silver in a solid sample is determined gravimetrically by converting the silver to Ag+ (aq) and precipitating it as silver chloride. Failure to do which of the following could cause errors in the analysis?I Account for the mass of the weighing paper when determining the mass of the sampleII Measure the temperature during the precipitation reactionIII Wash the precipitateIV Heat the AgCl precipitate to constant mass(A) I & II
(B) I & IV
(C) II & III
(D) I, III, & IV

Answer 1

Answer:

D

Explanation:

I - You should account the mass of the weighing paper to reduce it from the total mass at the end of the process, having only the mass of the Silver Chloride.

II - The precipitation of the silve chloride will occur independently of the temperature, because the Kps of this salt is very low (Ksp = [Ag+] .[Cl-]).

III - Washing the precipitate will secure the purity of the final product, it won't allow any other contaminant to be in your precipitante which could change your final mass.

IV - You should heat you AgCl precipitate so it will be dry, because of that the mass you will obtain is only the mass of the weighing paper and the silver chloride and nothing else.

Answer 2

Answer:(D) I, III, & IV

Explanation:

Firstly, the precipitate needs to be washed to remove water soluble impurities adsorbed on the surface of the precipitate which could affect its mass. Secondly, the precipitate must be heated to constant mass to drive off solvent molecules that adhered to the solid and could inflate the mass of the precipitate thereby introducing error to the analysis. The mass of the weighing paper must be accounted for because the mass read from the balance is the mass of the sample+ weighing paper.