# 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

General Chemistry Online! How many molecules are present in a given volume of gas at STP?