gọi halogen : `X_2`

`a.PT TQ:Ca+X_2 \overset(t^o)(->) CaX_2`

`b.n_{X_2}=n_{CaX_2}`

`=>{16,8}/{22,4}={83,25}/{40+2.X}`

`=>X=35,5(am u)`

`=>X:Cl`

vậy halogen : `Cl_2`

$\rm PTHH: Ca + X_2 \xrightarrow{to} CaX_2$

$n_{X_2}=\dfrac{16,8}{24,79}≈0,7(mol)$ 

theo pt 

$n_{CaX_2}=n_{X_2}$ 

`<=>`$\dfrac{83,25}{40+2M_X}=0,7$ 

`<=>`$28+1,4M_X=83,25$

`<=>`$M_X=39,46$ không có giá trị thỏa mãn 

sửa đề `đktc` 

$n_{X_2}=\dfrac{16,8}{22,4}=0,75(mol)$ 

theo pt 

$n_{CaX_2}=n_{X_2}$ 

`<=>`$\dfrac{83,25}{40+2M_X}=0,75$ 

`<=>`$M_X=35,5(g/mol)$ 

vậy `X:Cl` (chlorine)