Question on PV=nRT and moles

[Hexon]

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from kaplan's practice section q40 for general chem.
" a scuba tank is filled with 0.32 kg O2 gas that's compressed to a volume of 2.8L. If the temperature of the tank equilibriates with the water at 13C, what's the pressure inside the tank?"

i understand that it's an ideal gas law prob; PV=nRT
P=nRT/V

where
V=2.8L
T=13C+273K=286K
R=0.0821 L atm/mol K
n=20 moles of oxygen

now when i looked at the answers i saw that instead of using 20 moles of oxygen, they used 10 moles; is this a mistake or does oxygen's diatomic-ness influence the calculations?

thanks guys

 

[Donald Juan]

The molecular mass of O2 is 32 g/mol. It is no mistake.

 

[Hexon]

The molecular mass of O2 is 32 g/mol. It is no mistake.

oh i see where i went wrong; careless mistakes pfft