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