0.6 g of a sample of pyrolusite was boiled with 200 " mL of " `(N)/(10)` oxalic acid and excess of dilute sulphuric acid. The liquid was filtered and the residue washed. The filtrate and washing were mixed and made up to 500 mL in a measuring flask. 100 " mL of " this solution required 50 " mL of " `(N)/(30)KMnO_4` solution. Calculate the percentage of `MnO_2` in the sample `(Mn=55)`.

Explanation

`Mn^(4+)+2e^(-)to[Ew(MnO_2)=(87)/(2)=43.5]`
Total oxalic acid used `=200xx(1)/(10)=20mEq`
Excess of oxalic acid.
`=50xx(1)/(30)`
`=(5)/(3)mEq.` Of `KMnO_4` in `100 mL`
`=(5)/(3)xx5=(25)/(3)mEq`. Of `KMnO_4` in 500 mL
`=(25)/(3)mEq`. of oxalic acid in 500 mL
Oxalic acid used `=20-(25)/(3)=(35)/(3)mEq`
`=(35)/(3)mEq` of `MnO_2`
`=(35)/(3)xx10^(-3)xx43.5 g` of `MnO_2`
`=0.5075 g` of `MnO_2`
`% of MnO_2=(0.5075xx100)/(0.6)=84.583%`