Themes > Science > Chemistry > Inorganic Chemistry > More Information About Gas Laws > Gas Index > Gas Stoichiometry

As with any other type of chemical reaction, basic stoichiometry principles apply to gas phase reactions. Since the volume of a gas and the number of moles of that gas are directly related, we can do problems where we want to figure out the volume of gas produced instead of the weight or number of moles.

For example, hydrogen burns in oxygen to form water vapor.

2H2(g) + O2(g) -> H2O(g)
How much hydrogen must be burned to form 1.00 L of water vapor at 1.00 atm pressure and 3000C?

To work this problem, we need to figure out how many moles of water vapor we need to form. Use the ideal gas equation to compute this:

PV = nRT
1.00 atm * 1.00 L = n*0.0821 (L*atm/mol*K)* 473 K
n = 0.0258 moles
Thus, we need to form 0.0258 moles of water. Now it's just a basic stoichiometry problem: for each mole of water formed, you need 2 moles of hydrogen gas, so 0.0258 moles water * 2 moles H2/1 mole water = 0.0516 moles of hydrogen

Example: The gas acetylene burns according to the following equation

2C2H2(g) + 5O2(g) -> 2H2O(g) + 4CO2(g)
If you have a 10.0 L tank of acetylene at 25.0oC and 100 atm pressure, how much CO2 will you form if you burn all the acetylene in the tank?

Solution: First, use the ideal gas equation to determine how much acetylene gas you have

PV = nRT
100 atm * 10.0 L = n * 0.0821(L*atm/mol*K) * 298 K
n = 40.9 moles acetylene
Now that we have the moles of gas, it's the same as any other stoichiometry problem: 4 moles of CO2 are formed for every two moles of C2H2
40.9 moles C2H2 * 4 moles CO2/2 moles C2H2 = 81.8 moles CO2


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