\(n_{Na_2O}=\dfrac{12,4}{62}=0,2mol\\ a,Na_2O+H_2O\rightarrow2NaOH\\ NaOH+HCl\rightarrow NaCl+H_2O\\ n_{HCl}=n_{NaCl}=0,2.2=0,4mol\\ b,C_{M_{HCl}}=\dfrac{0,4}{0,2}=2M\\ c,m_{ddNaCl}=200.1,05+12,4=222,4g\\ C_{\%NaCl}=\dfrac{0,4.58,5}{222,4}\cdot100=10,52\%\)