A Brief Introduction to the Term Molarity
In this section, the simplest definition and Molarity formula will be explained with proper examples. You will find out how this formula is derived and how it can be used in different ways. After studying this section, you will find it convenient to understand the Molarity definition and formula properly. It is an important term used in different chapters. Hence, learning this formula will become absolutely mandatory.
Students have been taught various units of measurement, be it in Mathematics, Physics, or Chemistry as well. When it comes to Chemistry, there are various units of measuring different substances - their volume, concentration, etc.,- and one such unit which is widely used in Chemistry is the 'mole'.
Now, what is a mole? A mole is a unit of measurement used for a chemical substance from which comes the term 'Molarity'. Molarity, also known as the molar concentration of a solution is the technique of calculating the amount of substance a particular chemical solution contains. It is measured by considering two indicators, that is, the number of moles that are present in the solute and the volume of the solution. The volume of the solution is essentially calculated in the unit of liters. Molarity is symbolized by 'M'.
Definition of Molarity (M)
Molar Concentration or Molarity is defined as the number of moles of solute present in a definite amount of liters of the solution, that is, moles per liters of a solution.
Difference between Solute, Solvent, and Solution
Before going ahead, let us know the difference between the terms 'solute', 'solvent, and 'solution' to make it easier to understand the concepts that are going to come ahead. A solution can be defined as a homogeneous mixture that contains one or more solutes in it, which means that solutes are nothing but materials that are present in solutions. A solvent is a substance that can dissolve a solute. Therefore, what we call a solution is nothing but a solute that is dissolved into a solvent, forming a solution.
Molality (m) - A Brief Introduction
The term Molality was first found in a publication that goes by the name "Thermodynamics and the Free Energies of Chemical Substances", which was published in the year 1923 by G N Lewis and M Randall. It is symbolized by 'm'.
Definition of Molality (m)
Molality can be defined as the number of moles of a solute that can be found in a kilogram of the solvent, that is, moles per kilogram of a solvent.
Difference between Molarity and Molality
Though the terms Molarity and Molality may seem somewhat similar as the two words sound almost the same, the sharp contrast between them and their usage makes it very important for the students to understand and to know, as it creates a huge difference while using it for various calculations. It may be difficult to notice and sometimes escape the eyes of the students, but there is one important distinction between the two terms Molarity and Molality.
Molarity is the calculation of the number of solutes that are present in a liter of a solution. As we have mentioned earlier, a solution contains both the solute and the solvent. Whereas, Molality is the calculation of the number of solutes that are present in a kilogram of the solvent itself. As we already know, a solvent is something that dissolves the solute.
Mole Fraction - Definition
As we know, a solution is a mixture of various constituents, and the Mole Fraction, which can also be called the Molar Fraction, is defined as the amount of one particular constituent, which is expressed in terms of moles divided by the total amount of all the constituents present in the solution.
Weight Percentage - Definition
The Weight Percentage, which can also be termed as the Mass Percentage is nothing but the ratio of the mass of a solute and the mass of a solution, multiplied by 100.
Easy Way to Define Molarity with Formula
There are different terms used in the advanced syllabus of Chemistry. NCERT Chemistry has a set of physical chemistry where you will find a lot of new terms included and explained in every chapter. These terms are sometimes interlinked and have close meanings too. One such common term introduced in the advanced level chemistry by NCERT is molarity. Along with this term, you will also learn what morality and normality are.
Molarity is a new term for students who have just entered the advanced segment of chemistry. Different types of units will be introduced in Physical Chemistry. These units are used to measure the concentration of solutions and different constituents of a mixture.
Molarity is defined as the concentration of solute in a solvent. You can refer to it as the easiest way to represent the concentration of a solute in a solution. To understand the actual definition of this term, you will have to understand what molecular weight is. The first step is to understand that the total weight of a molecule of a solute is called its molecular weight.
The atomic weight of every constituent atom of the molecule is added to get the total weight of the solute’s molecule. When this molecular weight is expressed in grams, it represents one mole of that particular substance. This is what you have learned in your previous classes.
Let us recapitulate once more. One mole of a substance is the molecular weight of that substance expressed in grams. After learning what mole is, we can proceed to find out what the formula for Molarity is and how it is determined.
How Is the Correct Formula of Molarity Determined?
The correct formula of Molarity can only be determined when you know the proper definition of the term and the meaning of all the other chemical terms used to build the formula. As per the definition, the molarity of a solution is the total number of moles of solute present in a particular volume of solution.
If we represent everything with symbols then,
M = n/V
Here, ‘M’ stands for molarity, ‘n’ represents the number of moles of solute present in the solution and ‘V’ represents the volume of solution present in a container.
Now that you have studied what mole stands for, you can easily calculate the amount of solute present in a particular solution. After learning the Molarity definition and formula, you should try some examples that show how molarity can vary and how it can be determined. These examples will also help you to find out other terms associated with the formula. In a nutshell, every problem related to molarity can be solved once you get familiar with the term and its formula.
Examples Used to Explain Molarity
The next section of this description page will take you to the explanation of examples. In this section, you will learn how the formula mentioned above can be used to calculate other associated terms. The explanation of the Molarity formula with examples will help you grab the concept better as the experts have used simple language.
Let us consider an example to keep this interesting discussion running. If one mole of calcium carbonate (CaCO3) is 100g. It means that the molecular weight of calcium carbonate is 100. Moving on, if this amount of calcium carbonate is present in 4 liters of a solution, then molarity will be:
M = n/V
= ¼
= 0.25 mol/liters
You can understand how this calculation using the formula of Molarity has been done. Do not forget to put on the unit of molarity after the calculation is done.
If you consider another example to understand the Molarity ka formula, you will learn a new trick. There are different kinds of solutions you will study in advanced-level chemistry. When the solute and solvent are both liquids, do not forget to consider the resultant volume of the solution. For instance, when 1 liter of sulphuric acid is mixed in 2 liters of water, the solution will become 3 liters. If you know what is the formula of Molarity, then you will understand that the volume of both liquids will be considered.
The use of the Molarity formula in Chemistry is exemplary. You will find it in different chapters of inorganic chemistry too. Hence, you should learn the definition and Molarity formula properly using the explanation given by the experts.
Conclusion
This is all about the concept of molarity and its difference from molality. Make sure you understand both terms well so that you can identify the right formula to derive the solutions to problems easily. Focus on the examples given here to understand molarity better.
FAQs on Molarity
1. What is Molarity in chemistry?
Molarity (M), also known as molar concentration, is a measure of the concentration of a solute in a solution. It is defined as the number of moles of solute dissolved in exactly 1 litre (L) of the solution. It provides a quantitative way to express how much of a substance is present in a given volume of solution.
2. What is the standard formula for calculating Molarity?
The standard formula to calculate molarity is: M = n / V. In this formula:
- M represents the Molarity of the solution (in mol/L).
- n represents the number of moles of the solute.
- V represents the total volume of the solution in litres (L).
3. What are the units of Molarity?
The standard SI unit for Molarity is moles per litre, which is written as mol/L or mol·L⁻¹. It is also commonly denoted by a capital letter 'M', called "molar". For example, a 2 mol/L solution is often referred to as a "2 molar" or "2 M" solution.
4. How does Molarity differ from Molality?
The primary difference lies in what the solute is measured against.
- Molarity (M) is the number of moles of solute per litre of solution (solute + solvent).
- Molality (m) is the number of moles of solute per kilogram of solvent only.
5. What does it mean if a solution is labelled as '1 M'?
A solution labelled as '1 M' (read as "one molar") means it has a molarity of one. This specifically indicates that there is exactly one mole of solute dissolved in every one litre of the total solution. For instance, a 1 M solution of NaCl contains 58.44 grams (1 mole) of NaCl dissolved in enough water to make 1 litre of solution.
6. Why does Molarity change with temperature while Molality remains constant?
This difference is due to the denominator in their respective formulas. Molarity is defined by the volume of the solution (M = moles/volume). Volume is a physical property that expands or contracts with changes in temperature. Therefore, as temperature increases, the volume of the solution increases, causing the Molarity to decrease. In contrast, Molality is defined by the mass of the solvent (m = moles/mass), and mass does not change with temperature.
7. How is the molarity of a solution affected by dilution?
When a solution is diluted by adding more solvent, the number of moles of solute remains the same, but the total volume of the solution increases. This causes the molarity to decrease. The relationship between the initial and final molarity during dilution is described by the dilution equation: M₁V₁ = M₂V₂, where M₁ and V₁ are the initial molarity and volume, and M₂ and V₂ are the final molarity and volume.
8. In which experimental scenarios is Molality a more suitable unit of concentration than Molarity?
Molality is preferred over Molarity in experiments involving significant temperature changes or when dealing with colligative properties. Because Molality is based on the mass of the solvent, it is temperature-independent. This makes it more accurate for:
- Studies of colligative properties like boiling point elevation and freezing point depression.
- Thermodynamic calculations where temperature variations can affect the volume of the solution.
- Precise experiments in physical chemistry where consistency across different temperatures is crucial.
9. What is the relationship between Molarity and Normality for a given solution?
Normality (N) is another unit of concentration related to Molarity (M) by the n-factor (or valency factor) of the solute. The n-factor represents the number of equivalents per mole. The relationship is given by the formula: Normality (N) = Molarity (M) × n-factor. For an acid, the n-factor is its basicity; for a base, it's its acidity; and for a redox agent, it's the number of electrons transferred per mole.
10. How can you calculate the molarity of a pure substance, like water?
To calculate the molarity of a pure substance like water (H₂O), you need its density and molar mass. The density of water is approximately 1000 g/L. The molar mass of water is about 18.015 g/mol.
- First, find the number of moles (n) in 1 L of water: n = Mass / Molar Mass = 1000 g / 18.015 g/mol ≈ 55.5 mol.
- Since this is the number of moles in 1 litre (V=1 L), the Molarity (M) is n/V.
