IB Chemistry S1.5.3 Molar Volume of an Ideal Gas Notes

Understanding Molar Volume and Graphical Relationships in Gases

You’re inflating a balloon for a party.

With each breath, it grows larger, and you might start to wonder: how much space does the air inside actually take up?

What if you could measure the volume of the gas and count the number of molecules inside?

This curiosity leads us to an important concept in chemistry: molar volume.

What is Molar Volume?

Definition

Molar volume

The molar volume of a gas is the volume occupied by one mole of an ideal gas under specific conditions of temperature and pressure.

This property is a cornerstone of gas behavior, as described by the ideal gas law.

At STP (Standard Temperature and Pressure):

Temperature = 273.15 K (0°C)
Pressure = 100 kPa
Molar Volume = 22.7 dm³ mol⁻¹ (at 273.15 K and 100 kPa)

In simple terms, this means that one mole of any ideal gas will occupy 22.7 dm³ of space under these conditions, regardless of its chemical identity.

Example question

Suppose you have 2 moles of oxygen gas (O₂) at STP. What is the total volume of the gas?

Solution

Using the molar volume at STP:$$ \text{Volume} = \text{Moles} \times \text{Molar Volume}$$ $$\text{Volume} = 2 \, \text{mol} \times 22.7 \, \text{dm}^3 \ \text{mol}^{-1} = 45.4 \, \text{dm}^3$$
Thus, 2 moles of oxygen gas occupy 45.4 dm³ at STP.

Tip

Always confirm that the temperature and pressure match STP conditions when using the molar volume of 22.7 dm³ mol⁻¹.
If the conditions differ, use the ideal gas equation to calculate the volume.

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IB Chemistry S1.5.3 Molar Volume of an Ideal Gas Notes

Understanding Molar Volume and Graphical Relationships in Gases

What is Molar Volume?

Graphical Relationships Between Temperature, Pressure, and Volume

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Understanding Molar Volume and Graphical Relationships in Gases

What is Molar Volume?

Graphical Relationships Between Temperature, Pressure, and Volume

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Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry S1.5.3 Molar Volume of an Ideal Gas Notes

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

Understanding Molar Volume and Graphical Relationships in Gases

What is Molar Volume?

Graphical Relationships Between Temperature, Pressure, and Volume

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Excepteur sint occaecat cupidatat non proident

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Understanding Molar Volume and Graphical Relationships in Gases

What is Molar Volume?

Graphical Relationships Between Temperature, Pressure, and Volume

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