Igniting MnO_(2) inair converts it quantitativelyto Mn_(3) O_(4) . A sample of pyrolusite isof the followingcomposition : MnO_(2) = 80%, SiO_(2) and otherconstituents - 15% restbeingwater. Thesample is ignitedin air to constantweight,what is thepercentageof Mnin theignited smaple ?

Igniting MnO_(2) inair converts it quantitativelyto Mn_(3) O_(4) . A s

1.	Igniting MnO_(2) inair converts it quantitativelyto Mn_(3) O_(4) . A sample of pyrolusite isof the followingcomposition : MnO_(2) = 80%, SiO_(2) and otherconstituents - 15% restbeingwater. Thesample is ignitedin air to constantweight,what is thepercentageof Mnin theignited smaple ?
Answer» Solution :Suppose the weight of pyrolusite is x g wt. Of `MnO_(2)""( 80)/( 100) XX x = 0.8 x ` Wt. of `SiO_(2) ` etc. = `(15)/(100) xx x = 0.15 x` Wt. of WATER ` = (5)/( 100) xx x = 0.05 x` Whenpyrolusite isignited, `MnO_(2)` changes to `Mn_(3)O_(4)and H_(2)O` evaporates. Theresidue CONTAINS, therefore , `SiO_(2), ` etc., and `Mn_(2) O_(4)` Now , we know `MnO_(2) to Mn_(2) O_(4)` 0.8x g ApplyingPOAC for Mn atoms, moles of Mn is `MnO_(2)` = molesof Mnin `Mn_(3) O_(4)` `1 xx` moles of `MnO_(2) = 3 xx ` moles of `Mn_(3) O_(4)"" . . . (i)` `(0.8)/( 87) = 3 xx ("wt.of "Mn_(3) O_(4))/( 229) [{:(MnO_(2)=87),(Mn_(3)O_(4)=229):}] ` Wt. of `Mn_(3)O_(4) = 0.702` x g `:.` wt. of the residue= wt. of `Mn_(3)O_(4)` + wt. of `SiO_(2)`, etc. `= 0.702 x + 0.15 x = 0.852 x g ` Now, since Mn atoms areconserved, moles of Mn= moles of Mn = `Mn_(3) O_(4)` = moles of Mn in `MnO_(2)` `= 1 xx ` moles of `MnO_(2)` `= (0.8x)/( 87) (MnO_(2) = 87)` `:. ` wt. of Mn = molesof Mn ` xx ` at . wt. of Mn `= (0.8 x)/( 87) xx 55 g` `%`of Mn in residue = `("wt. of Mn")/( "wt. of residue ") xx 100` `= (0.8 x 55)/(87) xx (100)/( 0.852 x) = 59.37% `