Molar Volume (Edexcel GCSE Chemistry)

• At room temperature and pressure, the volume occupied by one mole of any gas was found to be 24 dm³ or 24,000 cm³
• This is known as the molar gas volume at RTP
• RTP stands for “room temperature and pressure” and the conditions are 20 ºC and 1 atmosphere (atm)
• From the molar gas volume the following formula triangle can be derived:

The Molar Volume Formula Triangle

Formula triangle showing the relationship between moles of gas, volume in dm³ and the molar volume

• If the volume is given in cm³ instead of dm³, then divide by 24,000 instead of 24:

Formula triangle showing the relationship between moles of gas, volume in cm³ and the molar volume

Moles into Volume

• Here are some examples of how you can manipulate the relationship to find the volume of gas from a given number of moles

Volume = Moles x Molar Volume

Examples of Converting Moles into Volumes Table

Volume into Moles

• Rearranging the formula allows you to calculate the amount of gas in moles from a given volume at RTP

Moles = Volume ÷ Molar Volume

Examples of Converting Volumes into Moles Table

Reacting Masses into Gas Volumes

• Sometimes in problem-solving we have to convert between the mass of a gas and its volume
• This is a two step problem which requires first converting the mass into moles and then from moles into gas volume
• The following example illustrates this: