Nồng độ SO₂ đã phản ứng là: $4.80\%=3,2M.$

                                        ${\text{2SO}}_{\text{2}}\left(g\right){\text{ + O}}_{\text{2}}\left(g\right)\stackrel{V_2{O}_5,{\text{ 450}}^o{\text{C - 500}}^{o}C}{\rightleftharpoons }{\text{ 2SO}}_3\left(g\right)$

Ban đầu:                              4                     2                                         0 (M)

Phản ứng:                           3,2                    1,6                                                           3,2                                  (M)

Cân bằng:                           0,8                    0,4                                                     3,2                                  (M)

${\text{K}}_{\text{C}}\text{=}\frac{{\text{[}S{O}_3\text{]}}^2}{{\text{[}{O}_2\text{].[}S{O}_2\text{]}}^2}=\frac{3,2^{\text{2}}}{\text{0,4}\times {\text{0,8}}^2}=40$