Đáp án:

 `M_{A} = 2,759 . 29 = 80 g/mol`

Gọi `x` là hoá trị của `S`

`%m_{S} = 32/(32 + 16.x) .100 = 40%`

`-> x = 3`

`-> SO3`

$Đáp$ $án$ $+$ $Giải$ $thích$ $các$ $bước$ $giải$ $:$

$Gọi$ $CTTQ$ $của$ $khí$ $A$ $là$ $(S_x O_y)_n$

$\text{%S}$ $=$ $\text{40%}$

$⇒$ $\text{%O}$ $=$ $\text{100%}$ $-$ $\text{40%}$ $=$ $\text{60%}$

$⇒$ $\frac{x}{y}$ $=$ $\frac{\text{40%}}{32}$ $:$ $\frac{\text{60%}}{16}$ 

$⇒$ $\frac{x}{y}$ $=$ $1,25$ $:$ $3,75$ 

$⇒$ $\frac{x}{y}$ $=$ $1$ $:$ $3$ 

$⇒$ $x$ $=$ $1$ $và$ $y$ $=$ $3$

$⇒$ $A$ $có$ $dạng$ $:$ $(SO_3)_n$ 

$\overline{M_A}$ $=$ $2,759$ $×$ $29$ $=$ $80$ $(g/mol)$

$⇒$ $($ $32$ $+$ $16$ $×$ $3$ $)$ $×$ $n$ $=$ $80$ 

$⇒$ $80n$ $=$ $80$ 

$⇒$ $n$ $=$ $1$

$⇒$ $CTHH$ $của$ $A$ $:$ $SO_3$

$Chúc$ $bn$ $hk$ $tốt$ $!!$ $:))))$

$Xin$ $ctlhn$ $ạ.$