Okay, to do this you have to work with the relative molecular mass (RMM). You can get this from looking at the periodic table.
The RMM for the whole molecule is:
58.933 + (2 x 35.453) + (12 x 1.008) + (6 x 15.9994) = 237.96
Then you work out the RMM for water:
(2 x 1.008) + 15.9994 = 18.0154
As there are 6 moles of water in this molecule then multiply the RMM of H2O by 6 = 108.0924
Finally, divide the total H2O RMM by the total molecule RMM and multiply by 100 to get a percentage:
(108.0925 / 237.96) x 100 = 45.42%
