Gas Volumes

Section: Understanding Gas Volumes and Molar Volume

• In chemistry, the volume of a gas is usually measured at standard temperature and pressure (STP), which is 0°C and 1 atmosphere pressure.
• At STP, one mole of any gas occupies approximately 22.4 litres. This is often referred to as the molar volume.
• It’s important to remember that this molar volume is regardless of the type of gas, be it hydrogen, oxygen, or carbon dioxide.
• The relationship between the volume of gas and the amount in moles can be expressed by the equation V=nV_m, where V is the volume of the gas, n is the number of moles and V_m is the molar volume.

Section: Avogadro’s Principle

• Avogadro’s principle states that equal volumes of gases at the same temperature and pressure contain the same number of molecules.
• Therefore, under the same conditions of temperature and pressure, one mole of gas A will occupy the same volume as one mole of gas B.
• Avogadro’s principle explains why the molar volume of a gas is the same for all gases at the same temperature and pressure.

Section: Calculating with Gas Volumes

• When working with gases in chemical reactions, often you need to calculate the gas volume involved.
• To do so, you can use the equation: V = nV_m. For example, the volume of 2 moles of oxygen gas at STP is 2 x 22.4 L = 44.8 L.
• On other occasions, you may be given the volume and asked to calculate the number of moles. In this case, you can rearrange the equation: n = V / V_m.

Section: Gas Volumes in Balancing Equations

• Chemical equations need to be balanced for both mass and volume, especially when working with gases.
• According to Avogadro’s principle, the ratio of volumes of different gases in a reaction is given by the ratio of their coefficients in the balanced chemical equation.
• For example, in the equation 2H2 + O2 ➔ 2H2O, it tells you that 2 volumes of hydrogen react with 1 volume of oxygen to produce 2 volumes of water. This approach is referred to as the volume ratio method.