Question

Potassium permanganate is prepared from the mineralpyrolusite, `MnO_(2)`. Its crystals have deep purple colour. It acts as an oxidizing agentin the neutral, alkaline as well as acidic medium. In acidic medium , it is used in volumetric analysis for estimation of ferrous salts, oxalates etc. The titrations are carried out in presence of `H_(2)SO_(4)`. However ,before using it as a titrant, it is first standized with standard oxalic acid solution or Mohr salt solution. In one of the experiments on titration , 13.4 g of dry pure sodium oxalate ( molar mass `=134 g mol^(-1)` ) was dissolved in 100mLof distilled waterand then 100 mL of 2M `H_(2)SO_(4)` were added. The solution, was cooledto `25.30^(@) C`. Now to this solution , 0.1 M `KMnO_(4)` solution was added till a very faint pink colour persisted.
If of 0.1 M `K_(2)Cr_(2)O_(7)` solution were used in place 0.1 M `KMnO_(4)` in presence of `H_(2)SO_(4)` in each case, the volume of 0.1 M`K_(2)Cr_(2)O_(7)` solution used would be
A. 200 mL
B. 400 mL
C. 333.3 mL
D. 666.6 mL

Answer 1

If `K_(2)Cr_(2)O_(7)` were used, the reaction would have been
`Cr_(2)O_(7)^(2-) + 16 H^(+) + 3C_(2)O_(4)^(2-) rarr 2Cr^(3+) + 7H_(2)O + 6CO_(2)`
Moles of `K_(2)Cr_(2)O_(7)` required for 0.1 mol of sodium oxalate `= (1)/(3) xx 0.1= 0.03333`. Hence , volume of 0.1 M `K_(2)Cr_(2)O_(7)` solution required `= 33.3mL`