$1000$ kg of the pyrite ore containing $85$ wt$.\%$ of FeS$2$ and $15$ wt$.\%$ of waste rock, is

oxidized in $100\%$ excess of air. The reaction is as follows:

$4$ FeS$2+11$ O$2==2$ Fe$2$O$3+8$ SO$2$

Waste does not react with oxygen and does not release any volatile components. The

residuum after the oxidation contains $2$ wt$.\%$ of FeS$2.$ Calculate: a$)$ Fe content in the

residuum $($in wt$.\%),$ b$)$ the volume of air used at $20\backslash$deg C and $0\mathrm{.}0981$ MPa, c$)$ the volume

and composition $($in mol.$\%)$ of the outgoing gas at $300\backslash$deg C and $0\mathrm{.}0981$ MPa, d$)$

conversion of FeS$2$