a,
\(KX+AgNO_3\rightarrow AgX+KNO_3\)
\(n_{AgNO3}=0,012\left(mol\right)=n_{AgX}\)
\(\Rightarrow M_{AgX}=\frac{2,82}{0,012}=235=108+X\)
\(\Rightarrow X=127\left(I\right)\)
Vậy halogen là iot
b,
\(n_{KI}=n_{AgNO3}=0,012\left(mol\right)\)
\(\Rightarrow C\%_{KI}=\frac{0,012.166.100}{25}=7,968\%\)