Gas stoichiometry at standard temperature and pressure

What volume is occupied by 4.21 moles of ethane, C2H6, (a) at standard temperature and pressure and (b) at the given temperature and pressure of Example 14.8, 8°C and 0.568 atm  

You can solve this problem with dimensional analysis, using the molar volume of a gas at STP, 22.4 L/mol. Plan Part (a) and answer the question. 

Now solve Part (b) using the molar volume you calculated in Example 14.8, which was 40.6 L/mol. 

7 Given a chemical equation, or a reaction for which the equation can be written, and the mass or number of moles of one species in the reaction, or the STP volume of a gaseous species, find the mass or number of moles of another species, or the STP volume of another gaseous species. 

Being able to convert between gas volume at STP and number of moles means we can expand Equation 10.2 to 

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There are ways other than density to include volume in stoichiometry problems. For example, if a substance in the problem is a gas at standard temperature and pressure (STP), use the molar volume of a gas to change directly between volume of the gas and moles. The molar volume of a gas is 22.41 L/mol for any gas at STP. Also, if a substance in the problem is in aqueous solution, then use the concentration of the solution to convert the volume of the solution to the moles of the substance dissolved. This procedure is especially useful when you perform calculations involving the reaction between an acid and a base. Of course, even in these problems, the basic process remains the same change to moles, use the mole ratio, and change to the desired units. 

Visuahzing a mole as a pile of particles, however, is just one way to understand this concept. A sample of a substance has a mass, volume (generally used with gases), and number of particles that is proportional to the chemical amount (measured in moles) of the sample. For example, one mole of oxygen gas (O2) occupies a volume of 22.4 L at standard temperature and pressure (STP 0°C and 1 atm), has a mass of 31.998 grams, and contains about 6.022 X 10 molecules of oxygen. Measuring one of these quantities allows the calculation of the others and this is frequently done in stoichiometry. 

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