Question

Copper can be determined gravimetrically by precipitation as copper(I)thiocyanate, CuSCN. A wrought aluminum-copper sample(4.5167 g) was dissolved in a minimum amount of concentrated HCl followd by the additon of 25 ml of sulfurous acid to reduced the copper. This solution was then diluted with 100ml of deionized water, heated to near boiling and an excess of 10% ammonium thiocyanate, NH4SCN, solution was added. The Precipitate was allowd to settle overnight, then filtered trough a pre-weighed porcelain filter, washed an dried to constan weight at 110 C. The mass of the empty crucible as 23.7157 g and the mass of the crucible with the CuSCN precipitated was 24.0664g.
a) If the maximum % Cu in the sample could be no more than the % Cu in pure CuCl2, calculate the maximun possible mass of Cu in the sample.
b) If you needed to make up a total volume of 100ml of the 10% ammonium thiocyanate solution, give a detailed description of how you would make up this solution in the lab including both the calculations and the lab procedure. NH4SCN is a solid
c) Assuming the maximum amount of Cu in the sample (as calculated in a), calculate the volume of the ammonium thiocyanate solution you would need to add with a 10% excess. Assume an ammonium thiocyanate solution density of 1.00g/ml
d) Calculate the % Cu in the sample given in the problem