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How can balanced equations be used to calculate the volume of gases formed in chemical reactions?
Question Date: 2015-04-30
Answer 1:

Balancing a chemical equation is equivalent to the law of conservation of mass. That is, you are saying that the same amount of stuff on the products has to come from the same amount of stuff on the reactants, plus or minus some energy. I like to think of it in terms of individual atoms that are being shifted around to form new bonds, and how they are shifted around is determined by how energetically favorable it is. An example is the reaction between methane and oxygen .

The thing to remember is that when you balance a chemical equation, you are preserving the number of each species. This number, on an experimentally measurable scale, is moles. A mole is about 6.02 x 1023 of something just like a dozen is 12 of something or a baker's dozen is 13 of something. The number of moles tells you how many of a certain species you have; this is also known as Avogadro's number.

To convert it into a physical property such as volume you need a conversion factor. There is a rule of thumb that an ideal gas at STP (standard temperature and pressure- i.e., around 273 K and 1 atm) will occupy 22.4 L/mol. It's important to remember that this only holds for an ideal gas, where there are no intermolecular forces and collisions between atoms is elastic, at a certain temperature or pressure; otherwise you need to use the ideal gas law to back-out the volume at a different temperature or pressure. A noble gas, for instance, could be reasonably considered as an ideal gas. Knowing the number of moles of your product or reactant, the temperature, the pressure, and the above conversion factor, you now have what you need to figure out the volume of gas from unit analysis.

Why is it that we can say any ideal gas will take up 22.4 L/mol? This has to do with the fact that a gas is mostly empty space. Unlike in a solid in which the atoms are closely packed together, the atoms in a gas are spaced so far apart that the size of the atom does not matter.

What if the gas is not an ideal gas? Then you will need a more sophisticated way of estimating the volume that accounts for intermolecular forces (e.g., Van der Waals or hydrogen bonding) and inelastic scattering. Generally for larger intermolecular forces or more inelastic scattering, you would expect a smaller volume. You can see several modifications to the ideal gas law in an attempt to account for this (mostly empirically) here .

Hope this helps!

Answer 2:

Any molecule in a gas will occupy the same amount of volume, regardless of how big the molecule itself is (this is what makes gasses different from solids or liquids). As a result, balanced chemical equations will tell you how many molecules are created or consumed, and the resulting volume will scale accordingly.

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