\(M_A=17.2=34\left(g/mol\right)\)
=> A là H2S
\(n_S=\dfrac{6,4}{32}=0,2\left(mol\right)\); \(n_{H_2S}=\dfrac{4,48}{22,4}=0,2\left(mol\right)\)
PTHH: 7Mg + 9H2SO4 --> 7MgSO4 + S + H2S + 8H2O
1,4<--------------------------0,2
=> mMg = 1,4.24 = 33,6 (g)