`m_{MgO}=20g`
`=> m_{HgO}=52,55-20=32,55g`
`=> n_{HgO}=\frac{32,55}{217}=0,15(mol)`
`n_{MgO}=\frac{20}{24+16}=0,5(mol)`
`a)` `HgO+H_2\overset{t^o}{\to}Hg+H_2O`
`n_{H_2}=n_{HgO}=0,15(mol)`
`V_{H_2}=0,15.22,4=3,36(l)`
`%m_{MgO}=\frac{20.100%}{52,55}\approx 61,44%`
`%m_{HgO}=100%-61,44%=38,56%`
`n_{Hg}=n_{H_2}=0,15(mol)`
`=> m_{Hg}=201.0,15=30,15g`
`b)` `Zn+2HCl\to ZnCl_2+H_2`
`n_{Zn}=n_{H_2}=0,15(mol)`
`=> m_{Zn}=0,15.65=9,75g`