For solving the question we use the ideal gas law:
pressure × volume = moles × gas constant × temperature
Because you have the pressure in torr the gas constant will have the following value:
gas constant = 62.363 ( L × torr ) / ( K × mol )
To mach the unit of the constant the temperature have to be expressed in kelvin:
T°K = T°C + 273
T°K = 27 + 273 = 300 °K
Now we add the values in the formula:
pressure × volume = moles × gas constant × temperature
800 × 4.89 = moles × 62.363 × 300
moles = (800 × 4.89) / (62.363 × 300)
moles = 0.209
The we construct the following reasoning:
if 1 mole of oxygen contains 6.022 × 10²³ molecules
then 0.209 moles of oxygen contains X molecules
X= (0.209 × 6.022 × 10²³) / 1 = 1.26 × 10²³ oxygen molecules