Which scientific tool is most useful when trying to convert a known amount of grams of water to moles of water?

The periodic table

Explanation:

The periodic table can show the atomic number and atomic mass of any atoms easily. To convert grams of water to moles of water, you need to know the atoms that made up water molecules then find out their molecular mass. Water is made of two hydrogens and one oxygen atoms. Then you can find the atomic mass of hydrogens is 1 while oxygen is 16. The molecular mass of water will be 2*1 + 16 = 18g/mol.