$\text{X gồm:} K_2MnO_4, MnO_2$
$n_{KMnO_4}=\dfrac{15,8}{158}=0,1(mol)$
$\text{Phương trình:}$
$2KMnO_4\xrightarrow{t^o}K_2MnO_4+MnO_2+O_2$
0,1____________0,05________0,05________0,05 $\text{(mol)}$
$\to m=m_{K_2MnO_4}+m_{MnO_2}$
$\to m=197.0,05+87.0,05=14,2g$
$V_{O_2}=0,05.22,4=1,12(l)$