`a)`

`MgCO_3+2HCl->MgCl_2+CO_2+H_2O`

`R_2(CO_3)_n+2nHCl->2RCl_n+nCO_2+nH_2O`

`n_{CO_2}={3,36}/{22,4}=0,15(mol)`

`n_{HCl}=2n_{CO_2}=0,3(mol)`

`m_{dd\ HCl}={0,3.36,5}/{7,3\%}=150(g)`

`m_{dd\ sau}=14,2+150-0,15.44=157,6(g)`

`->n_{MgCO_3}=n_{MgCl_2}={157,6.6,028\%}/{95}=0,1(mol)`

`n_{CO_2}=n_{MgCO_3}+n.n_{R_2(CO_3)_n}`

`->n_{R_2(CO_3)_n}={0,05}/n(mol)`

`->M_{R_2(CO_3)_n}=2R+60n={14,2-0,1.84}/{{0,05}/n}=116n`

`->R=28n`

`n=2;R=56->FeCO_3`

`\%m_{MgCO_3}={0,1.84}/{14,2}.100\%=59,15\%`

`\%m_{FeCO_3}=100-59,15=40,85\%`

`b)`

`n_{FeCl_2}=n_{FeCO_3}=0,05(mol)`

`MgCl_2+2NaOH->Mg(OH)_2+2NaCl`

`FeCl_2+2NaOH->Fe(OH)_2+2NaCl`

`Mg(OH)_2` $\xrightarrow{t^o}$ `MgO+H_2O`

`4Fe(OH)_2+O_2` $\xrightarrow{t^o}$ `2Fe_2O_3+4H_2O`

`->n_{MgO}=n_{MgCl_2}=0,1(mol);n_{Fe_2O_3}=1/2n_{FeCl_2}=0,025(mol)`

`->m_{CR}=0,1.40+0,025.160=8(g)`