What is the molar concentration of a solution of pure water?

Concentration (mol/dm³) = (density (g/dm³)) / (molecular mass (g/mol))

The density of water = 1000 g/dm³

The molecular mass of H₂O = (21 g/mol) + (116 g/mol) = 18 g/mol

Concentration mol/dm³ = 1000 g/dm³ / 18 g/mol ~ 55.5 mol/dm³