\(n_{Na_2CO_3}=\dfrac{10\%.265}{106}=0,25\left(mol\right)\\ PTHH:Na_2CO_3+CaCl_2\rightarrow CaCO_3\downarrow+2NaCl\\ a,n_{CaCO_3}=n_{Na_2CO_3}=0,25\left(mol\right)\\ m_{kết.tủa}=m_{CaCO_3}=0,25.100=25\left(g\right)\\ b,n_{NaCl}=2.0,25=0,5\left(mol\right)\\ m_{NaCl}=0,5.58,5=29,25\left(g\right)\\ m_{ddsau}=m_{ddNa_2CO_3}+m_{ddCaCl_2}-m_{CaCO_3}=265+500-25=740\left(g\right)\\ C\%_{ddNaCl}=\dfrac{29,25}{740}.100\%\approx3,953\%\)