Tough AP chemistry homework problem.. please help

Tetrodotoxin is a toxic chemical found in Fugu pufferfish, a popular but rare delicacy in
Japan. This compound has an LD50 (the amount of substance that is lethal to 50% of the
population sample) of 10. µg per kg of body mass. Tetrodotoxin is 41.38% carbon, 13.16%
nitrogen, 5.37% hydrogen, and the remaining amount consisting of oxygen. What is the
empirical formula of Tetrodotoxin? If 3 molecules of Tetrodotoxin have a mass of
1.59x10^(-21) g, what is the molecular formula? What number of molecules would be the LD50
for a person weighing 165 lbs? 1 kg=2.2046lb

100 - 41.38 - 13.16 - 5.37 = 40.09% oxygen
Take a hypothetical 100 gram sample of Tetrodotoxin.
(41.38 g C) / (12.0108 g/mol) = 3.44523262 moles C
(13.16 g N) / (14.0067 g/mol) = 0.939550358 mole N
(5.37 g H) / (1.0079 g/mol) = 5.32790951 moles H
(40.09 g O) / (15.9994 g/mol) = 2.50571896 moles O
Divide by the smallest number of moles:
C: 3.44523262 / 0.939550358 = 3.66689512
N: 0.939550358 / 0.939550358 = 1.00000000
H: 5.32790951 / 0.939550358 = 5.67070138
O: 2.50571896 / 0.939550358 = 2.66693418
In order to turn these ratios into integers, it looks to be necessary to multiply by 3:
C: 3 x 3.66689512 = 11.0006854
N: 3 x 1.00000000 = 3.0000000
H: 3 x 5.67070138 = 17.0121041
O: 3 x 2.66693418 = 8.00080254
Round to the nearest integer. The empirical formula is:
C11N3H17O8
One molecule of Tetrodotoxin has the mass 1.59x10^(-21) / 3 = 5.3x10^(-22) g
One mole of Tetrodotoxin weighs 5.3x10^(-22) g x 6.022x10^23 = 319.166 g
The molecular mass of C11N3H17O8 is 319.2693 g/mol, so C11N3H17O8 is also the molecular formula.

(165 lbs) x (1kg / 2.2046 lb) x (10 µg/kg) = 748.43509 µg = 748.43509x10^(-6) g
(748.43509x10^(-6) g) / (319.2693 g/mol) x (6.022x10^(23) molecules/mol) =
1.4 × 10^18 molecules of Tetrodotoxin