A sample of pynolusite `(MnO_(2))` weights 0.5 gm. To this solutioin 0.594 gm `As_(2)O_(3)` and a dilute acid are added. After the reaction has stopped `As^(+3)` is `AS_(2)O_(3)` is titrated with 45 mlof M/50 `KMn_(4)` solution. Calculate the percentage of `MnO_(2)` in pyrolusite.

Options

A. `65.25%`
B. `68%`
C. `67.3%`
D. `66.6%`

Answer

A

Explanation

`MnO_(2)+As_(2)O_(3)rarrMn^(+2)+AsO_(4)^(-3)`
For excess of `As_(2)O_(3)`, we have
`As_(2)O_(3)+MnO_(4)^(-)rarrMn^(+2)+AsO_(4)^(-3)`
Npw `As_(2)O_(3)` getting oxidized to `AsO_(4)^(-3)`, the half reaction is
`As_(2)O_(3)+5H_(2)Orarr2As+10H^(+)+4e^(-)`
for 1mol of `As_(2)O_(3)`, No. of `e^(-)s=4`,
So, eq. wt of `Al_(2)O_(3)=198//4`
M.eq of `As_(2)O_(3)=(0.594)/(198//4)xx1000=12`
Me eq of excess of `As_(2)O_(3)` = M.eq of `KMnO_(4)`
`=45=((1)/(50)xx5)=4.5`
The other half reaction is
`MnO_(4)^(-)+5e^(-)+8H^(+)rarrMn^(2)+4H_(2)O`
M.eq of `As_(2)O_(3)` used for `MnO_(2)=12-4.5=7.5`
`therefore` wt implies `(W)/(87//2)xx1000=7.5impliesW=0.326gm`
`%MnO_(2)=(0.326)/(0.5)xx100=65.25%`