An oxygen sample has a volume of 4.89 l at 27 °c and 800.0 torr. how many oxygen molecules does it contain?

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