Dhristi JEE 2022-24

What is Avogadro's Law?

Avogadro’s law is a gas law and is also referred to as Avogadro’s hypothesis or Avogadro’s principle. It states that the total number of molecules or atoms of a gas is directly proportional to the volume that the gas occupies at a constant temperature and pressure. This law is much related to the ideal equation of the gas because it links the temperature, volume, pressure and the amount of substance for a gas. 

This law is named after Amedeo Carlo Avogadro, an Italian scientist, who had suggested that two different gases that occupy the same amount of volume at constant pressure and temperature must have equal numbers of atoms or molecules. 

In this article, you would the notes for Avogadro law class 11 which includes what is Avogadro’s law, Avogadro's law definition, Avogadro's hypothesis, state Avogadro’s law, Avogadro's law formula, and Avogadro's law example.

Avogadro's Law Formula

At constant pressure and temperature, the Avogadro’s law is expressed as follows:

V ∝ n

\[\frac{V}{n}\] = k

Here, V is the volume of the gas 

n is the amount of gaseous substance expressed in moles, and

k is constant 

When the amount of the gaseous substance is allowed to increase, the increase in the volume that is occupied by the gas is calculated as follows:

\[\frac{V_1}{n_1}\] = \[\frac{V_2}{n_2}\] = k , according to Avogadro’s law

It is graphically represented as follows.

(Image will be uploaded soon)

(Image will be uploaded soon)

Here, the straight line that indicates that both the quantities are directly proportional tends to pass through the origin, which implies that zero moles of gas occupy zero volume.

Derivation of Avogadro’s Law

Avogadro’s law is derived from the ideal gas equation, which is expressed as follows:

PV = nRT

Where in,

P refers to the pressure exerted by a gas on its container wall

V refers to the volume occupied by gas

n is the total amount of gaseous substance or number of moles of the gas

R refers to the universal gas constant, and

T refers to the absolute temperature of the gas

After rearranging the ideal gas equation, 

\[\frac{V}{n}\] = \[\frac{(RT)}{P}\]  in which,

the value of \[\frac{(RT)}{P}\] is constant because the temperature and pressure are kept constant and the product of two or more constants is a constant. Hence, 

\[\frac{V}{n}\] = k

Molar Volume of a Gas

According to Avogadro’s law, the ratio of the volume and amount of gaseous substance is constant at constant pressure and temperature and is denoted by k. This constant is determined by

k = \[\frac{(RT)}{P}\] 

Under standard temperature and pressure,

T = 273.15 K and P = 101.325 kiloPascals

Hence, the volume of one-mole gas at STP is,

\[\frac{(8.314 J.mol-1.K-1)(273.15 K)}{(101.325 kPa)}\] = 22.4 litres

Hence, one mole of any gas occupies 22.4 litres volume at STP.

Avogadro’s Law Example

The respiration process is an example of Avogadro’s law. When we inhale, there is an increase in the molar quantity of the air in our lungs. This leads to an increase in the lungs volume. Take a look at the image below.

(Image will be uploaded soon)

(Image will be uploaded soon)

Limitations of Avogadro’s Law

Even though it is perfectly applicable to all the ideal gases, Avogadro’s law only provides the relationships for the real gases. The deviation of the real gases from the ideal behaviour tends to increase at higher pressures and lower temperatures. Gaseous molecules that have lower molecular masses like hydrogen and helium follow Avogadro’s law to a good extent when compared to the heavier molecules.

Solved Example on Avogadro’s Law

Example 1: A tyre that consists of 10 moles air and occupies 40L volume loses half of the volume because of a puncture. What is the amount of air left in the deflated tyre at STP?


Initial amount of air  n1 is 10 mol

Initial volume of tyre V1 is 40 L

Final volume of tyre V2 is 20 L

According to Avogadro’s law, 

the amount of air left in the deflated tyre,

n2 = \[\frac{V_{2}n_{1}}{V_1}\]  = 5 moles

Hence, the deflated tyre contains 5 moles air.

Important Points to Remember

  • Equal volumes of various gases have an equal number of molecules under similar conditions of temperature and pressure.

  • Avogadro law is only for gas.

  • Avogadro law is important because the amount of gas and volume is detected by using this law. 

  • An example of the Avogadro law is the deflation of automobile tyres.

  • The value of the constant can be determined with the help of an equation  k = (RT)/P

  • Experts use the Avogadro law more in Chemistry to measure the pressure or the number of gases that exist in the vessel.

  • This is also used by the chemical and the process engineers for taking the calculations.

  • Gases with molecules that have less low molecular mass like helium and hydrogen obey the Avogadro law than the gases with higher molecular masses.

  • The process of respiration is another example of Avogadro law.

  • When the temperature and pressure are the same or constant then the amount increases then the volume increases.

  • When the temperature and pressure are the same or constant then the amount decreases then the volume also decreases.

  • Avogadro law is named after a famous Italian scientist Amedeo Carlo Avogadro.

Tips to secure Better Marks in the Exams regarding This Subject

  • Pay Attention - Students should pay attention in the classes where the chapter is covered by the teacher and try to make notes while listening to the teacher.

  • Read the Chapter - After the chapter is covered read the chapter and try to understand the topic while referring to the notes made during the class.

  • Clear Your Basics - Physics is all about the basics and these should be cleared from the beginning like from earlier classes so that they won't face problems when they reach Class 10,11 and 12. Always keep your basics strong so that the students can understand the topics which will be taught in the future because those concepts will help them to understand the other chapters.

  • Memorize the Basics - here the basics means the formulas or equations which are in physics because there are problems that have to be solved by applying the formulas and in order to do so students need to know which formula to be used to solve the questions. Some students mix up the formulas in different problems; this leads to having the wrong solutions. Equations are important to solve the practicals so it has to be memorized.

  • Derivation of Formulas - students should also keep in mind how the equation is derived because that will help them to understand why the equation will work in the problem which will give a step to have the right answer to a problem.

  • Attend the Problems in the Exam - Some students leave the questions which have problems in it but they fail to understand that that is the best way to score marks in exams because the marks will be given step-wise if the student has answered it accordingly. In order to achieve this one has to practice the problems when they have time.

  • Practice Makes Perfect - In Physics solving the problems is important so students should not avoid practicing the problems from the exercises given in the book and they should also practice the examples given in the book on their own this will make them understand the solutions. They will know why the equations are used in a question. 

  • Doubts Clearing - Always try to be curious to know more, this will create doubts in the mind regarding any topic and write it down in the notes they can ask the teachers, by this students get to learn many new things in the process.

  • Make Notes - Always try to make notes by referring to the book or the study materials they have, sometimes they get extra information which can be useful in exams.

  • Extra Guidance - Physics is a subject where students are recommended to have extra guidance to study like taking extra tuitions for this subject because some students understand more easily and they also clear their doubts with their tutors.

  • Double-Check the Answers - Always try to solve the questions with the proper understanding of the problem and after that double-check the answers which are given in the solution books. Students can download the solutions books from the Vedantu website. 

Vedantu has the facility for online tuitions for Physics and they can enroll for the tuitions, they will be provided with the best teachers of the subjects they want, they will be provided with the study materials so that they won't have to worry about the notes.

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FAQs (Frequently Asked Questions)

1. State and explain Avogadro’s law.

According to Avogadro’s law, equal volumes of gases at constant pressure and temperature tend to occupy the same number of molecules. If the mass of the ideal gas is given, the volume and the number of moles present in the gas are proportional directly at a constant temperature and pressure. This relation is derived from the kinetic theory of the gases when there is an assumption of an ideal gas. The Avogadro’s law is valid approximately for the real gases at sufficiently higher temperatures and lower pressures.

2. State Avogadro’s hypothesis.

According to Avogadro's hypothesis, two samples of the gas having equal volume, at a constant temperature and pressure, consist of the same number of gaseous molecules. With the help of Avogadro's hypothesis, chemists are able to predict the behaviour of the ideal gasses. Amedeo Avogadro derived the hypothesis in the year 1811. Elaborating on the hypothesis, he quoted that the volume of the given gas does not tend to depend on the size of the mass of the molecules of the particular gas. Avogadro's hypothesis was then applied in the Avogadro's law for denoting that the volume of a gas, V is equal to the constant, k, multiplied by the number of the moles of the gas, which is given by, V=kn.