3.1

a)
$2\ HI\left(g\right)\xrightleftharpoons[]{}H_2\left(g\right)+I_2\left(g\right)$
b)
$K_c=\frac{\left[H_2\right]\ \cdot\left[I_2\right]}{\left[HI\right]^2}=\frac{\left(4{,}79\cdot10^{-4}mol\right)^2}{\left(3{,}53\cdot10^{-4}mol\right)^2}=1{,}8412875...\approx1{,}84$
$K_c=\frac{\text{mol/dm}\cdot\text{mol/dm}^3}{\left(\text{mol/dm}^3\right)^2}=1$