$\alpha \% = 20\%$`->`$\alpha = 0.2$

$\rm HNO_2 \rightleftharpoons NO_2^- +H^+$   $\rm K_a=\dfrac{[NO_2^-]\times[H^+]}{[HNO_2]}$

Giả sử phân ly được: $\rm [NO_2^-]=[H^+]=\alpha C_0(M)$

$\rm [HNO_2]=C_0-\alpha C_0=(1-\alpha)C_0(M)$

Ta có biểu thức: $\rm K_a=\dfrac{[NO_2^-]\times[H^+]}{[HNO_2]}$

Hay $\rm \dfrac{(\alpha \times C_0) \times (\alpha \times C_0)}{(1-\alpha)C_0}=4.6 \times 10^{-4}$

`<=>`$\rm \dfrac{\alpha^2 \times C_0}{1-\alpha}=4.6 \times 10^{-4}$

`<=>`$\rm \dfrac{0.2^2 \times C_0}{1-0.2}=4.6 \times 10^{-4} $

`<=>`$C_0=\dfrac{4.6  \times 10^{-4} \times (1- 0.2)}{0.2^2}=0.0092(M)$