Key Difference – Combined Gas Law vs Ideal Gas Law
When studying about different gases, the relationships between volume, pressure, the temperature of the gas and the amount of gas present is very important. These relationships are given by the ideal gas law and the combined gas law. When explaining these laws, the term “ideal gas” is often used. An ideal gas does not exist in reality but is a hypothetical gaseous compound. It has no intermolecular forces between gas molecules. However, some gases can behave as an ideal gas when proper conditions (temperature and pressure) are provided. The gas laws are created for ideal gases. When using these gas laws for real gases, some corrections are considered. The combined gas law is a combination of three gas laws; Boyle’s Law, Charles’ Law, and GayLussac’s Law. The key difference between combined gas law and ideal gas law is, the combined gas law is a collection of three gas laws whereas ideal gas law is an individual gas law.
CONTENTS
1. Overview and Key Difference
2. What is Combined Gas Law
3. What is Ideal Gas Law
4. Relationship Between Combined Gas Law and Ideal Gas Law
5. Side by Side Comparison – Combined Gas Law vs Ideal Gas Law in Tabular Form
6. Summary
What is Combined Gas Law?
Combined gas law is formed from the combination of three gas laws; Boyle’s Law, Charles’ Law, and GayLussac’s Law. The combined gas laws indicate that the ratio of the product of pressure and volume and the absolute temperature of a gas is equal to a constant.
PV/T = k
In which P is pressure, V is volume, T is temperature and k is a constant. When the combined gas law is used along with Avogadro’s law, it results in the ideal gas law. The combined gas law has no owner or discoverer. The above relationship can be given as below as well.
P1V1/T1 = P2V2/T2
This gives the relationship between volume, temperature and pressure of an ideal gas at two states. Therefore, this equation can be used to explain and predict these parameters at an initial state or final state.
Boyle’s Law
At constant temperature, the volume of an ideal gas is inversely proportional to the pressure of that gas. This means the product of initial pressure (P1) and initial volume (V1) is equal to the product of final pressure (P2) and final volume (V2) of the same gas.
P1V1 = P2V2
Charles’ Law
At constant pressure, the volume of an ideal gas is directly proportional to the temperature of that gas. This law can be given as below.
V1/T1 = V2/T2
GayLussac’s Law
At constant volume, the pressure of an ideal gas is directly proportional to the temperature of the same gas. This can be given as below,
P1/T1 = P2/T2
What is Ideal Gas Law?
The ideal gas law is a fundamental law in chemistry, and it indicates that the product of pressure (P) and volume (V) of an ideal gas is directly proportional to the product of temperature (T) and a number of particles of gas (n).
PV = kNT
Here, k is a proportionality constant. It is known as Boltzmann’s constant. The value of this constant is found to be 1.38 x 10^{23} J/K. However, the ideal gas is simply expressed as follows.
PV = nRT
Where n is the number of moles of gas present, and R is the universal gas constant given by 8.314 Jmol^{1}K^{1}. This equation can be used only for ideal gases. If it needs to be used for real gases, some corrections are made because real gases have many exceptions from ideal gases.
This new equation is known as van der Waals equation. It is given as below.
(P + a{n/V}^{2}) ({V/n} – b) = RT
In this equation, “a” is a constant that depends on the type of gas and b is also a constant which gives the volume per mole of gas (occupied by the molecules of gas).
What is the Relationship Between Combined Gas Law and Ideal Gas Law?
 When the combined gas law is used along with Avogadro’s law, it results in the ideal gas law.
What is the Difference Between Combined Gas Law and Ideal Gas Law?
Combined Gas Law vs Ideal Gas Law 

The combined gas law is formed from the combination of three gas laws; Boyle’s Law, Charles’ Law, and GayLussac’s Law.  The ideal gas law is a fundamental law in chemistry; it indicates that the product of pressure (P) and volume (V) of an ideal gas is directly proportional to the product of temperature (T) and a number of particles of gas (n). 
Formation  
The combined gas law is formed by the combination of Boyle’s Law, Charles’ Law, and GayLussac’s Law.  The ideal gas law is an individual law. 
Equation  
The combined gas law is given by PV/T = k  The ideal gas law is given by PV = nRT 
Summary – Combined Gas Law vs Ideal Gas Law
Gas laws are used to understand and predict the behaviour and properties of a gas. The difference between combined gas law and the ideal gas law is, the combined gas law is a collection of three gas laws whereas ideal gas law is an individual gas law. The combined gas law is formed from Boyle’s Law, Charles’ Law, and GayLussac’s Law.
Reference:
1.Helmenstine, Anne Marie. “Understand the Combined Gas Law in Chemistry.” ThoughtCo. Available here
2.Helmenstine, Anne Marie. “What Is the Ideal Gas Law? Review Your Chemistry Concepts.” ThoughtCo. Available here
3.“How Does the Ideal Gas Law Differ from the Combined Gas Law?  Socratic.” org. Available here
Image Courtesy:
1.’Temperature Pressure Law’By Egmason – Own work, (CC BYSA 3.0) via Commons Wikimedia
2.’37147128632’by Travis Wise (CC BY 2.0) via Flickr
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