\(a)n_{Ba\left(NO_3\right)_2}=\dfrac{300.17,4\%}{100\%.261}=0,2mol\\ Ba\left(NO_3\right)_2+Na_2SO_4\rightarrow BaSO_4+2NaNO_3\\ n_{Na_2SO_4}=n_{BaSO_4}=n_{Ba\left(NO_3\right)_2}=0,2mol\\ C_{\%Na_2SO_4}=\dfrac{0,2.142}{250}\cdot100\%=11,36\%\\ b/n_{NaNO_3}=0,2.2=0,4mol\\ m_{ddNaNO_3}=300+250-0,2.233=503,4g\\ C_{\%NaNO_3}=\dfrac{0,4.85}{503,4}\cdot100\%\approx6,75\%\)
