Formula from Chapter 3: Dilution law
\(c_{\mathrm{1}} \cdot V_{\mathrm{1}} = c_{\mathrm{2}} \cdot V_{\mathrm{2}}\)
You can use this if: the only thing that happened was adding solvent to a solution
\(c_{\mathrm{1}}\) \(= 2.00\ \mathrm{M}\)
\(V_{\mathrm{1}}\) \(= 5.00\ \mathrm{mL}\)
\(V_{\mathrm{2}}\) \(= 50.0\ \mathrm{mL}\)
\(c_{\mathrm{2}}\) \(= c_{\mathrm{1}} \cdot \dfrac{V_{\mathrm{1}}}{V_{\mathrm{2}}}\)
\(\ \ \ =2.00\ \mathrm{M} \cdot \dfrac{5.00\ \mathrm{mL}}{50.0\ \mathrm{mL}}\)
\(\ \ \ =2.00\ \mathrm{M} \cdot 0.10000\)
\(\ \ \ =0.200\ \mathrm{M}\)