a)\(Ba+H2SO4-->BaSO4+H2\)
b)\(n_{H2SO4}=\frac{39,2}{98}=0,4\left(mol\right)\)
\(n_{Ba}=n_{H2SO4}=0,4\left(mol\right)\)
\(a=m_{Ba}=0,4.137=54,8\left(g\right)\)
c)\(Fe3O4+4H2--->3Fe+4H2O\)
\(n_{Fe}=\frac{3}{4}n_{H2}=0,3\left(mol\right)\)
\(m=m_{Fe}=0,3.56=16,8\left(g\right)\)