\(n_{O_2}=\dfrac{16.8}{22.4}=0.75\left(mol\right)\)
\(2KMnO_4\underrightarrow{t^0}K_2MnO_4+MnO_2+O_2\)
\(1.5..............................................0.75\)
\(m_{KMnO_4}=1.5\cdot158=237\left(g\right)\)
2KMnO4-to>K2MnO4+ MnO2+O2
n O2=16,8\32=0,525 mol
=>m KMnO4=0,525.2.158=165,9g