The mass of water is 18.87kg.
1st) It is necessary to convert the 5.00 gal into mL, knowing that 1 gal is equal to 1 qt, and 1 L is equal to 1.057 qt. Then we convert the L into mL:
[tex]5.00\text{gal}\cdot\frac{4qt}{1\text{gal}}\cdot\frac{1L}{1.057qt}\cdot\frac{1000mL}{1L}=18,921.5mL[/tex]2nd) Now, with the formula of density, we can calculate the mass of water replacing the values:
[tex]\begin{gathered} \text{density}=\frac{mass}{volume} \\ 0.997g\cdot mL^{-1}=\frac{mass}{18,921.5mL} \\ \text{mass}=0.997g\cdot mL^{-1}\cdot18,921.5mL \\ \text{mass}=18,864.7g \end{gathered}[/tex]3rd) Finally, it is necessary to convert the grams into kg:
[tex]18,864.7g\cdot\frac{1\operatorname{kg}}{1000g}=18.87\operatorname{kg}[/tex]So, the mass of water is 18.87kg.
