Partial pressure is found by Dalton's Law:
Px = PTotal ( nx / nTotal )
where
Px = partial pressure of gas x
PTotal = total pressure of all gases
nx = number of moles of gas x
nTotal = number of moles of all gases
Step 1Find PTotal
Standard pressure is 1 atm.
Step 2Add up the number of moles of the component gases to find nTotal
nTotal = noxygen + nnitrogen
nTotal = 100 + 900
nTotal =1000cm3
Step 3Now you have all the information needed to plug the values into the equation and solve for Poxygen
P-oxygen = PTotal ( n-oxygen / nTotal )
oxygen = 1 atm ( 100 / 1000 )
oxygen= 0.1atm