The number of molecules in one mole of a covalent substance is #"6.022 x 10"^23"#, known as Avogadro's number. In order to determine the number of molecules of water are in 6.25g of water, the mass of the water must be converted to moles.

This is done by dividing the mass of the water by the molar mass of water, which is #"18.015g/mol"#.

#"6.25g H"_2"O"# x #"1 mol"/"18.015g"# = #".347 mol H"_2"O"#

To determine the number of molecules, multiply the number of moles times #"6.022 x 10"^23"# molecules.

#".347 mol H"_2"O"# x #"6.022 x 10"^23# #"molecules/mol"# = #"2.09 x 10"^23# molecules of water.

#"6.25g H"_2"O"# contains #"2.09 x 10"^23# molecules of water.