\[[\mathrm{Cl^-}] = \frac {n_\mathrm{Cl^-,tot}}{V} = \frac {n_\mathrm{Cl^-,NaCl} + n_\mathrm{Cl^-,MgCl_2}}{V}\]
\[\begin{aligned}n_\mathrm{Cl^-,NaCl} &= n_\mathrm{NaCl} = \frac {m_\mathrm{NaCl}}{M_\mathrm{NaCl}} = \frac {1,00\mathrm{g}}{(22,99+35,45)\mathrm{g/mol}} = 0,01711157\mathrm{mol} \\ n_\mathrm{Cl^-,MgCl_2} &= 2n_\mathrm{MgCl_2} = 2\frac {m_\mathrm{MgCl_2}}{M_\mathrm{MgCl_2}} = 2 \cdot \frac {1,00\mathrm{g}}{(24,31+35,45 \cdot 2)\mathrm{g/mol}} = 0,01329610\mathrm{mol}\end{aligned}\]
\[[\mathrm{Cl^-}] = \frac {(0,01711157 + 0,01329610)\mathrm{mol}}{0,100\mathrm{dm}^3} = 0,38117764\text{mol/dm}^3 ≈ 0,381\mathrm{mol/dm^3}\]
