How many grams of sodium acetate is to be mixed with $\mathrm {1.0 ~ dm^3 ~ 0.4 }$ molar acetic acid solution to have hydrogen ion concentration, $\mathrm {[H^+] = 2.0 × 10^{-4} ~ giondm^{-3} }$ in the solution ? $\mathrm {[k_a = 1.8 × 10^{-5}].}$