Chemistry: The Molecular Science (5th Edition)

Molar mass of water $(H_2O)$ = 1.01 * 2 + 16.0 = 18.0 g/mol $$\frac{0.998 \space g}{1 \space mL} \times \frac{1000 \space mL}{1 \space L} \times \frac{1 \space mol}{18.0 \space g} = 55.4 \space mol/L$$