A sample of pyrolusite `(MnO_2)` weighs 0.5 g. To its solution, 0.6674 g of `As_2O_3` and dilute, acid are added. After the reaction has ceased, arsenic `(As^(3+))` in `As_2O_3` is titrated with `45mL` of `(M)/(50)` `KMnO_4` solution. Calculate the percentage of `MnO_2` in pyrolusite. (Atomic weight of As is 74.9 and that of Mn is 55.)

Explanation

m" Eq of "`KMnO_4=45xx(1)/(10)=4.5`
Unreacted `As_2O_3=4.5mEq`
`=4.5xx10^(-3)xx49.5g" of "As_2O_3`
`=0.222g`
`{:[(As_2O_3to2AsO_4^(3-)+10^(o+)+4e^(-)),(Ew(As_2O_3)=(197.80)/(4)=49.45):}]`
Amount of `As_2O_3` reacted `=0.6674-0.222`
`=0.4448g`
`=(0.4448)/(49.45)=0.0089 Eq" of "As_2O_3`
`[Eq(MnO_2)=(87)/(2)=43.5]`
`=0.0089xx45.5 g" of "MnO_2`
`=0.39 g" of "MnO_2`
`% of MnO_2=(0.39)/(0.5)xx100=78.2%`