Explanation:
Given that, density of water = 1 g/mL i.e., 1 g of water dissolves in 1 mL of water.
\({\rm{Molarity}}\left( {\rm{M}} \right){\rm{ = }}\frac{{{\rm{Given}}{\mkern 1mu} {\mkern 1mu} {\rm{mass \times 1000}}}}{{{\rm{Molar}}{\mkern 1mu} {\mkern 1mu} {\rm{mass \times Volume}}{\mkern 1mu} {\mkern 1mu} {\rm{in}}{\mkern 1mu} {\mkern 1mu} {\rm{mL}}}}\)
\({\rm{ = }}\frac{{{\rm{1 \times 1000}}}}{{{\rm{18 \times 1}}}}{\rm{ = 55}}{\rm{.5mol/L}}\)