How many molecules are present in a given volume of gas at STP?

1. Figure out how many moles of CO₂ gas you have. Assume the CO₂ is an ideal gas; you know the temperature and pressure (it's at STP) and you know the volume, so you should be able to compute the number of moles using the ideal gas equation of state: n = PV/RT = (1 atm)(0.475 L)/(0.08206 L atm mol^-1 K^-1 × 273.15 K) = 0.0211915 mol
2. Use Avogadro's number to convert moles into molecules: 0.0211915 mol × 6.02×10²³ molecules/mol = 1.28 × 10²² molecules

To find the volume of a gas sample, given the number of molecules, the pressure, and the temperature:

1. Convert molecules of gas to moles of gas, using the mole-to-molecule conversion.
2. Assuming the gas sample is ideal, you can easily compute the volume of the gas using the ideal gas equation of state because you know the temperature, pressure, and moles of gas.

Author: Fred Senese senese@antoine.frostburg.edu