A sample of 1500 mg of an alloy that contains silver, copper, and chromium is dissolved and the solution containing Ag+ , Cu2+, and Cr3+ ions, is diluted to exactly 500 cm3. One tenth of the volume of that solution is taken for further procedure: After elimination of silver and copper, chromium is oxidised in it according to the following unbalanced equation:
OH- + Cr3+ H2O2 CrO2-4 +H2O
Then 25.00 cm3 of a 0.100 molar Fe(II) salt solution are added. The following reaction (written in an unbalanced form) is taking place:
H+ Fe2+ CrO2-4 Fe3+ Cr3+ H2O
According to the unbalanced equation:
H+ +Fe2+ +MnO-4 Fe3+ +Mn2+ +H2O
a volume of 17.20 cm3 of a 0.020-molar KMnO4 solution is required for an oxidation of the Fe(II) salt which remains unoxidized in the solution. In another experiment, a volume of 200 cm3 of the initial solution is electrolysed. Due to secondary reactions, the efficiency of the electrolysis is 90 % for metals under consideration. All three metals are quantitatively deposited in 14.50 minutes by passing a current of 2 A through the solution.
Balance the three chemical equations and calculate the composition of the alloy in % by mass.
Relative atomic masses: Ar(Cu) = 63.55; Ar(Ag) = 107.87; Ar(Cr) = 52.00