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

1. How many molecules are present in 475 mL of CO₂(g) at STP?
2. What is the volume occupied by 3.50 x 10²⁴ SO₂ molecules at STP? /OL>

   
   Julia

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

   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

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/gases/faq/molecules-per-volume-at-stp.shtml