Ta có: \(m_{dd}=300\cdot1,05=315\left(g\right)\) \(\Rightarrow C\%_{Na_2CO_3}=\dfrac{15,9}{315}\cdot100\%\approx5,05\%\)
Mặt khác: \(n_{Na_2CO_3}=\dfrac{15,9}{106}=0,15\left(mol\right)\) \(\Rightarrow C_{M_{Na_2CO_3}}=\dfrac{0,15}{0,3}=0,5\left(M\right)\)