# 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.