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Molarity is used to describe the number of reactants or products dissolved in one liter of solution. Molarity is a commonly used term when measuring the concentration of a solution.
- Solutions with more solute and less solvent are called Concentrated Solutions.
- Solutions with more solvent and less solute are known to be Dilute Solutions.
- It is often helpful to know how many moles of solute are present in one liter of solution, especially when the solutions are involved in chemical reactions.
Molarity Formula is given as:
| \(M = \frac{n (moles of solute) }{V (volume of solution)}\) |
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Key Terms: Molarity, Moles, Volume, Solute, Solvent, Solution, Molality, Molecular Formula, Molarity Formula, Molar Concentration
What is Molarity?
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Molarity is defined as the number of moles of solute contained in a liter of solution.
- Molarity is calculated by dividing the moles of solute by the volume of the solution.
- It is denoted by the symbol ‘M’.
- It is also referred to as Molar Concentration.
- The units of Molarity are given as M or mol or mol/L.
- Molarity is represented using the unit symbol mol/L or mol/dm3 in the SI unit.
Read More:
| Relevant Concepts | ||
|---|---|---|
| Hinsberg’s Reagent | Normality Formula | Moles to Grams |
| Mole Fraction | Types of Solutions | Difference between Molar Mass and Molecular Mass |
Molarity Formula
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Molarity Formula is given as follows:
| M = n/V |
Where
- M = Molarity of a particular solution
- n = Number of moles of the solute
- V = Volume of the solution
It can also be understood as
Molarity = Number of moles of the solute/Volume of solution in liters
| \(M = \frac{n (moles of solute) }{V (volume of solution)}\) |
Solved ExampleQues. Calculate the molarity of a given solution of sodium sulphate if the solution is prepared using 15 g of sodium sulphate and the volume of the solution is 125 ml. Solution: We know that
Thus, the molecular mass of sodium sulphate will be 23×2+32+16×4 = 142 Now, Number of moles of sodium sulphate in the given solution will be n = mass in grams/molecular weight = 15/142 = 0.106 Volume of the solution = 125 ml Converting the values in terms of litres, Volume of the solution = 125/1000 = 0.125 L Now, using the Molarity Formula: Molarity = Number of Moles of Solute/Volume of Solution Molarity = 0.106/0.125 = 0.85 M Therefore, the molarity of the given solution is 0.85M. |
Solutions Video Lecture
Factors Affecting Molarity
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Here are some major factors affecting Molarity:
Effect of Dilution on Molarity
To make a concentrated solution, it has to be diluted with the solvent its volume changes as a result. If a solution is diluted from V1 to V2, the molarity of the solution also changes. The changes are shown in the equation below
| M1V1 = M2V2 |
Moles of Solute in Original Solution 1 = Moles of Solute in Diluted Solution 2
In the above equation,
- M1= Initial Molarity of the solution
- V1 = The volume that is being transferred
- M2= Final Concentration of the solution
- V2 = Final total Volume of the solution
Effect of Temperature on Molarity
- The volume of a substance will usually be affected by a change in temperature.
- Since it is based on the volume of the solution, Molarity gets affected by temperature.
- According to this concept, when there is an increase in temperature, density will decrease.
- The volume increases when temperature increases and vice versa.
Applications of Molarity
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The applications of Molarity are as follows:
- Molarity is very useful for scientists and chemists who work in different fields of science.
- It is important in measuring the concentration of solutions.
- It helps to check the concentration of a particular solution and make chemical solutions before starting a chemical experiment.
- Molarity is also highly useful for many laboratory techniques (Eg: Analysing blood samples).
- These are some of the applications of molarity in the field of science.
Molarity and Molality
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Molarity can be defined as the number of moles of solute contained in a liter of solution whereas, Molality can be defined as the number of moles of solute contained in a kilogram of solvent.
- Molarity is represented by M whereas molality is represented by m.
- The SI unit for Molarity is mol/L and the SI unit for Molality is mol/Kg.
- Molarity is the ratio of several moles of the solute and the volume of the solution in liters.
- Molality is the ratio of several moles of the solute and the volume of the solute in kilograms.
Difference between Molarity and Molality
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Molarity and Molality are two common terms dealing with the Concentration of a Solution and are often confused with each other. Here is the difference between Molarity and Molality:
| Molarity | Molality |
|---|---|
| Molarity is the total number of moles of solute present per liter of solution. | Molality is the total moles of a solute present per kilogram of solvent. |
| It is given as M = number of moles of the solute /Volume of solution in liters. | It is given as m = Numbers of moles of solute/Mass of solvent in Kg. |
| It has a unit of mol L-1. | It has a unit of mol Kg-1. |
| It is dependent on the volume of the solution. | It is dependent on the mass of the solvent. |
| It is affected by the change in temperature. | It is not affected by the change in temperature. |
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Things to Remember
- Molarity is the number of moles of solute contained in a liter of solution.
- It is represented by the symbol M and its SI unit is mol/L.
- Molarity is the ratio of several moles of the solute and the volume of the solution in liters.
- Molarity Formula is given as M = nV = Number of Moles of the Solute/Volume of Solution in Litres.
- Molarity is found to be very useful for scientists and chemists in checking the concentration of various solutions.
- It is used in making up new solutions and is also used in pharmaceutical and laboratory settings.
Previous Year Questions
- The molarity of pure water is…
- What is the molarity of H2SO4 solution that has a density… (NEET 1996)
- The mass of one mole of a substance in grams is called its…
- In water-saturated air the mole fraction of water vapour is… (NEET 2019)
- The boiling point of 0.2 mol kg−1 solution of X… (NEET 2015)
- At 35∘C, the vapor pressure of CS2 is 512 mm Hg… (JEE Main - 2020)
- What is the mole fraction of the solute in a… (NEET 2015)
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- Two open beakers one containing a solvent and the other… (JEE Main - 2020)
- A solution of sodium sulfate contains 92g of… (JEE Main - 2019)
Sample Questions
Ques. Calculate the molarity of the solution if a solution is prepared by bubbling 1.56 grams of hydrochloric acid in water and the volume of the solution is 26.8 mL. (5 Marks)
Ans. We know that the chemical formula of hydrochloric acid is HCl and the chemical formula for Water is H2O/
- Molecular weight of HCl will be 35.5 ×1 + 1×1 = 36.5 moles/gram
- Molecular weight of H2O will be 1 ×2 + 16 ×1 = 18 moles/gram
Mass of hydrochloric acid in the solution is given as 1.56 g
Number of moles of hydrochloric acid = n/T= mass in grams/molecular weight
nT = 1.5636.5 = 4.27×10−2 mole
Volume of the solution is given as 26.8 mL
Calculating the volume in terms of litres,
Volume = 26.8/1000 = 2.68×10−2 Litre
Using the molarity formula,
Molarity = Number of moles of element/volume of solution in litres
Molality = 4.27×10−2 /2.68×10−2 = 1.59 M
Thus, the molarity of the solution is calculated as 1.59 M.
Ques. Diluting a solution does not change which of the following?
(A) concentration
(B) volume
(C) millilitres of solvent
(D) moles of solute (2 Marks)
Ans. (D) Moles of Solute
Explanation: When diluting a solution, the solvent's concentration, Volume, and Milli Liter change. But the moles of solute in a solution do not change.
Ques. If 5 litres of water are added to two moles of glucose to make a solution, the molarity? (3 Marks)
Ans. Using the molarity formula:
M = number of moles of the solute/volume of solution in litres
= 2/5
= 0.4 M
Thus, the molarity is calculated as 0.4 M.
Ques. What is the molarity of 600 mL of KI solution that contains 5.50 moles of the solute? (3 Marks)
Ans. According to the Molarity formula:
M = number of moles of the solute/volume of solution in litres
= 5.50 mols/0.6 L
= 9.16 M
Thus, the molarity is calculated as 9.16 M.
Ques. What volume of a 4.0 M solution would contain 15.5 moles of sodium thiosulfate? (3 Marks)
Ans. According to the Molarity formula:
M = number of moles of the solute/volume of solution in litres
Volume = No. of Moles/Molarity
= 15.5/4
= 3.9 Liters of the solution
Thus, the volume of the solution is 3.9 Liters.
Ques. The concentration of glucose in normal blood is approximately 90 mg per 100 mL. What is the molarity of glucose solution in blood? (3 Marks)
Ans. Mass of Glucose= 90mg = 90 / 1000 = 0.09 g
Moles of Glucose = 0.09 / 180 = 0.0005 mol
Volume of the solution = 100 mL = 1.10 L
M = number of moles of the solute volume of solution in litres
= 0.00050.10
= 0.0005 M
Ques. A person has a 5.0 M solution of hydrochloric acid (HCl) and his new experiment requires 150.0 mL of 2.0 M HCl. How much quantity of water and 5.0 M HCl should the scientist use to make 150.0 mL of 2.0 M HCl? (Use Molarity Formula) (5 Marks)
Ans. We know, C1V1 = C2V2
- C1 = Concentration of the solution
- V1 = Volume of the solution (and they are 5.0 M HCl)
- C2 = Concentration of the desired solution
- V2 = Volume of the desired solution (or 150.0 mL of the 2.0 M HCl solution)
HCl’s 5.0 M (V1) = HCl’s 2.0 M (150.0 (mL))
V1 = 60.0 mL of 5.0 M HCl
If we use 60.0 mL of 5.0 M HCl to make the solution, the amount of water we need to properly dilute the solution to the correct molarity and volume is as follows:
150.0 mL – 60.0 mL = 90.0 mL
Thus, in order to make 150.0 mL of 2.0 M HCl, he will need 60.0 mL of 5.0 M HCl and 90.0 mL of water.
Ques. Explain the properties of Molarity. (3 Marks)
Ans. The properties of Molarity are as follows:
- The sum of molarity gives the entire molar concentration, which is the density of the mixture divided by the molar mass of the mixture or the reciprocal of the molar volume of the mixture.
- The ionic strength is proportional to the sum of the molarity of salts in an ionic solution.
- Molarity is dependent upon the variation in the quantity of the solution caused due to thermal expansion.
Ques. State the difference between Solute, Solvent, and Solution. (3 Marks)
Ans. Solute and Solvent are two components of a Solution.
- Solution can be defined as a homogeneous mixture of one or more solutes in a solvent.
- Solute is a substance that can be dissolved into a solution by a solvent.
- Solvent is a chemical substance that dissolves a solute, resulting in a solution.
Ques. What is the Molarity Formula? (2 Marks)
Ans. Molarity Formula is given as follows:
M = n/V
Here, ‘M’ refers to molarity, ‘n’ is the number of moles of solute present in the solution, and ‘V’ represents the volume of solution present in a container.
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