When you have 100 g of compound, then based on the percentages given, there are 40.0 g C, 6.70 g H, and 53.3 g O. Convert those to moles:
C: 40.0 g / 12.0 = 3.33 moles of C
H: 6.70 g / 1.01 = 6.63 moles of H
O: 53.3 / 16.0 = 3.33 moles of O
Dividing by the smallest (3.33), we get a C:H:O mole ratio of 1:2:1
So, The empirical formula is CH2O.
Now, That formula has a molar mass of [12.0 + 2(1.0) + 16.0] = 30.0
And we are given it's molar mass is = 240
So, no. of units of CH2O = 240 / 30 = 8
8 x CH2O = C8H16O8, and that is the molecular formula.
[tex]C_8H_{16}O_8[/tex]
