Unformatted text preview: 5.4 Gas Stoichiometry 195 Table 5.2 Molar Volumes for Various Gases at o°C and 1 atm
Molar Volume (L) 22.397 22.402 22.433 22.434 22.397 22.260 22.079 Gas Oxygen (02) itrogen (N2) Hydrogen (H2) Helium (He) Argon (Ar) Carbon dioxide (C02) Ammonia (NH3) 5.11 22.42 L of a gas would just fit into this beach ball. Figure STP: aoc and 1 atm This volume of 22.42 liters is the molar volume of an ideal gas (at O'C and I atm). The measured molar volumes of several gases are listed in Table 5.2. Note that the molar volumes of some of the gases are very close to the ideal value, while others deviate significantly. Later in this chapter we will discuss some of the reasons for the deviations. The conditions aoc and I atm, called standard temperature and pressure (abbreviated STP), are common reference conditions for the properties of gases. For example, the molar volume of an ideal gas is 22.42 liters at STP (see Fig. 5.11). EXAMPLE 5.11 A sample of nitrogen gas has a volume of 1.75 L at STP. How many moles of N2 are present? Solution
We could solve this problem by using the ideal gas equation, but we can take a shortcut by using the molar volume of an ideal gas at STP. Since I mole of an ideal gas at STP has a volume of 22.42 L, 1.75 L N2 at STP will contain less than I mole. We can find how many moles using the ratio of 1.75 L to 22.42 L: 1.75 L-N2
X I mol N2 22.42 L-N2 = 7.81 X 10 -2 mol N2 SEE EXERCISES 5.63 AND 5.64 Many chemical reactions involve gases. By assuming ideal behavior for these gases, we can carry out stoichiometric calculations if the pressure, volume, and temperature of the gases are known. EXAMPLE 5.12 Quicklime (CaO) is produced by the thermal decomposition of calcium carbonate (CaC03). Calculate the volume of CO2 at STP produced from the decomposition of 152 g CaC03 by the reaction CaC03(s) ~ CaO(s) + CO2(g) ...
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