Number of Oxygen Molecules in a cylinder. Please help!!!

A tank of compressed oxygen gas at a doctor's office has a pressure of 103 times atmospheric pressure. If the volume of the tank is 655 cm3, how many oxygen molecules does it contain?

I used the equation PV = nRT

for P I used 103 atm, V I used 655 cm^3, R I used 8.314, and for temperature I assumed room temp so I used 300 K.

My answer for n was 2.74 moles * 6.022e23 particles = 1.65e24 molecules

however, this is wrong. I am not sure what I am doing wrong!

Since you used 8.314 as the ideal gas constant, the volume must be in cubic meters and the pressure must be in pascals

1 atmosphere = 1.013 * 10^5 Pascals
Pressure = 103 * 1.013 * 10^5
1 cm = 0.01 m
1 cm^3 = 0.01^3 = 1 * 10^-6 m^3
Volume = 655 * 1 * 10^-6 = 6.55 * 10^-4

(103 * 1.013 * 10^5) * 6.55 * 10^-4 = n * 8.314 * T

When using PV = nRT you have to get the units correct.

P must be in Pa (pascals). To convert atm to Pa, multiply by 101325 Pa (because 1atm = 101325 Pa).

V must be in m³. To convert cm³ to m³, divide by 10⁶ (because 1m³ = 100cm x 100cm x 100cm = 10⁶ cm³).

Your values for R and T are fine, as is conversion of moles to particles.

the only possible problem I could find is that room temperature isn't quite 300k (unless your instructor tells you such)

I would try using 295K as its more accurate or using whatever Room Temperature you are given and convert it to Kelvin.

u need to put v in litres if pressure is in atm