`a)PTHH:`
`Fe_3 O_4 + 8HCl` $\xrightarrow{t^o}$ `2FeCl_3 + FeCl_2 + 4H_2 O`
`79/292` `158/73` `(mol)`
`b)`
`n_[Fe_3 O_4]=[69,6]/232=0,3(mol)`
`n_[HCl]=79/[36,5]=158/73(mol)`
Có: `[0,3]/1 > [158/73]/8->Fe_3 O_4` dư
`=>m_[Fe_3 O_4(dư)]=(0,3-79/292).232=6,83(g)`