Moles in Gases: Molar Volume

Gases behave very differently from solids and liquids. You cannot simply weigh a gas on a balance in the same way you weigh a solid. Instead, chemists use the relationship between volume and moles to quantify gases — and this relationship is remarkably simple under certain conditions.

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Molar Volume at Room Temperature and Pressure (RTP)

At room temperature and pressure (RTP), defined as approximately 298 K (25 °C) and 101 kPa (1 atm), one mole of any gas occupies approximately 24.0 dm³ (24,000 cm³).

This value is called the molar volume and is the same for all gases at the same temperature and pressure (assuming ideal behaviour). It does not matter whether the gas is hydrogen, oxygen, carbon dioxide, or any other gas — one mole always occupies 24.0 dm³ at RTP.

The key equation is:

n = V / 24.0

Where:

• n = number of moles (mol)
• V = volume of gas (dm³) at RTP

Or equivalently: V = n × 24.0 and n = V / 24.0

If the volume is given in cm³, convert first: V (dm³) = V (cm³) / 1000.

Worked Example 1