\(n_{O_2}=\dfrac{20-14,24}{32}=0,18\left(mol\right)\\ Đặt:M\left(hoá.trị.x\right)\\ 4M+xO_2\rightarrow\left(t^o\right)2M_2O_x\\ n_{M_2O_x}=\dfrac{0,18.2}{x}=\dfrac{0,36}{x}\left(mol\right)\\ M_2O_x+2xHCl\rightarrow2MCl_x+xH_2O\\ n_{HCl}=\dfrac{0,36}{x}.2x=0,72\left(mol\right)\\ \Rightarrow V_{ddHCl}=\dfrac{0,72}{0,8}=0,9\left(lít\right)\)