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Molarity is defined as the mole concentration of any solution which is present in that solution. A mole is defined as the unit of measurement for the amount which is present in a substance having the same number of particles. A mole is the SI unit of an amount of a substance. The symbol of molarity is “M”. Molarity is one of the most used units for the measurement of concentration. Various reactions take place in a solution so it is very important to understand the unit of expression of the substance in the solution.
Read more: Le’ Chatelier's principle
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KeyTerms: solution, molarity, molality, mole fraction, mass percent, solute, solution
What is molarity?
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Molarity is defined as the number of moles of a solute that is being dissolved per liter of the solution. It is the unit of measurement of the moles present in the solution.
Or, it is defined as the number of moles of a solute that are present in 1 liter (1000ml) solution. Molarity is dependent on temperature.
It is expressed as:
Molarity = no. of moles/volume of solution (in liters)
Hence, the unit of expression is moles/liter
A solution, solute, and solvent
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Solution: A solution is defined as a homogenous mixture of different solutes which are being dissolved in a solvent.
Solute: A solute is defined as a substance which is being dissolved in a medium or a solvent
Solvent: A solvent is defined as a medium or material in which a substance or solute dissolves or expresses its physical state (solid, liquid, or gas).
Read more:
| Relevant Concepts | ||
|---|---|---|
| Types of solutions | Normality | Organic Solvent |
| Dalton’s law of partial pressure | Electron | Natural gas |
Mass percent
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Mass percent is also known as weight percent and it is represented as (w/w %). It is defined as the ratio of the mass of the solute to the mass of the solution which is given multiplied by 100. Mass percent is used to represent the concentration of solute present in the solution.
It is expressed as:
Mass percent = (mass of solute/ mass of solution) x 100
Molality
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Molality is defined as the number of moles of a solute that is present per kg of the solvent. The symbol of molality is "m".
It is expressed as:
Molality(m) = number of moles of solute/ mass of solvent (in kg)
Mole fraction
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Mole fraction is defined as the number of moles of a component to the total number of moles present in the solute and the solvent.
Or, mole fraction is also defined as the number of moles present in the solute of a specific component to the total number of moles present in the solution.
It can be stated as if a substance 'a' is dissolved in a substance 'b'. then the number of moles present is ‘na’ and ‘nb’ respectively.
Hence, the mole fraction of a and b can be expressed as:
Mole fraction of a= number of moles of a/ number of moles of the solution
Mole fraction of b= number of moles of b/ number of moles of the solution.
Hence, the equation formed for the mole fraction is expressed as:
The sum of all the mole fractions is equivalent to 1.
Difference between molality and molarity
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Molarity and molality may sound similar but they are two different terms with completely two meanings. Molarity is defined as the number of solutes present per liter of the solution. Whereas, molality is defined as the number of moles of solutes present in one kg of a solvent.
Molarity is dependent on temperature but molality doesn’t depend on temperature.
Things to remember
- Molarity is defined as the number of moles of the solute which are present in 1 liter of the solution.
- Mole is the SI unit of measurement of the amount of substance present.
- Molarity is denoted as “M” and it is expressed in moles/liter.
- The solution is defined as the homogenous mixture of different solute particles that dissolve in the solvent.
- Weight percent is defined as the mass of the solute to the mass of the solution multiplied by 100. It is represented as (w/w%).
- Molality is defined as the number of moles of the solute to the total mass of a solute (in kg). It is denoted as “m”. and is expressed in moles/kg.
- Mole fraction is defined as the number of moles of a component to the total number of moles present in the solute and the solvent.
- The Sum of all the mole fractions is equal to one.
- Molarity is dependent on temperature but molality does not depend on temperature.
Sample Questions
Ques. What is molarity? (2 marks)
Ans. Molarity is defined as the number of moles of the solute dissolved in one liter of the solution. It is expressed in moles/liters and is denoted as “M”.
The mathematical expression for molarity is:
M= n/vliters
Where M is molarity, n is the number of moles, and vliters is the volume of solution in liters.
Ques. Calculate the molarity of NaOH in the solution prepared by dissolving 5 g in enough water to form 500 mL of the solution. (3 marks)
Ans. As we know, Molarity (M)= no. of moles of solute/ volume of solution (in L)
= (Mass of NaOH/ Molar mass of NaOH) / 0.500 L
= (5 g/40 g) / 0.500 L
= 0.125 mol/ 0.500 L
= 0.25 mol-1
= 0.25 M
Ques. Is solvent interrelated to solute in a solution? (1 mark)
Ans. A solvent is a medium or material in which the solute particle dissolves or expresses the physical state (solid, liquid, or gas) to form a solution.
Ques. How to find the molar mass of the element or compound? (2 marks)
Ans. Molar mass is defined as the atomic mass of the element in g/mol. To calculate the molar mass of an element we need to multiply the atomic mass (in AMU) by the molar mass constant (1 g/mol). To calculate the molar mass of a compound you need to add all the atomic masses of the respective atoms.
Ques. A solution is prepared by the addition of 3 g of a substance to 12 g of water. Calculate the weight percent of the solution. (2 marks)
Ans. Weight percent of a = (mass of a / mass of solution) / 100
= {3 g / (3 g of a + 12 g of water)} / 100
= (3 g / 15 g) / 100
= 20%
Ques. What is mole fraction? (1 mark)
Ans. Mole fraction is also defined as the number of moles present in the solute of a specific component to the total number of moles present in the solution.
Ques. The density of 4 M solutions of NaCl is 1.25 g mL-1. Calculate the molality of the solution. (4 marks)
Ans. Given M= 4 mol L-1
Mass of NaCl in 1 L solution = 4 * 58.5 = 234
Mass of 1 L solution = 1000* 1.25
= 1250 g
Mass of water in solution = 1250 – 234
= 1016 g
Molality= no. of moles of solute/ mass of solvent in kg
= 4 mol/ 1.016 kg
= 3.938 m
Ques. What is the difference between molarity and molality? (2 marks)
Ans. Molarity is defined as the number of solutes present per liter of the solution. Whereas, molality is defined as the number of moles of solutes present in one kg of a solvent.
Ques. Calculate the molarity of ethanol in the water solution when the mole fraction of ethanol is 0.045. keep the density of water as one. (5 marks)
Ans. Molarity= no. of moles of solute/volume of solution
Calculate the no. of moles of water by taking the given mass of 1000 g
Given, the density of water= 1 kg/m
Hence, the water present is equal to 1 liter
The molar mass of water (H2O) = 2*1 +1*16
= 18 g/mol
No. of moles in 1 litre water = mass of water/ molar mass of water
= 1000/18
= 55.55 mol
Mole fraction of ethanol = no. of moles of ethanol/total no. of moles of the solution
But mole fraction of ethanol= 0.045 (given)
Using the values,
0.045 = no. moles of ethanol/ (no. of moles of ethanol + no. of moles of water)
0.045 = no. of moles of ethanol/ (no. of moles of ethanol + 55.55)
No. of moles of ethanol = 2.61 moles
Therefore,
Molarity = no. of moles of ethanol/ volume of solution
= 2.61 moles/ 1 liter
= 2.51 M
Ques. Calculate the molarity of the solution when 1.25 g of HCl is bubbled in water. The volume of the solution is 25 mL. (5 marks)
Ans. Molecular weight of HCl = 1*1 + 35.5 *1 = 36.5 mol/g
Molecular weight of water = 1*2 + 16*1 = 18 mol/g
Mass of HCl = 1.25 g
No. of moles of HCl = mass/ molecular weight
= 1.25/ 36.5
= 0.034 mol
Volume of solution in liters = 25/ 1000
= 0.025 Liters
Molarity = no. of moles of HCl/ volume of solution in liters
= 0.034/ 0.025
= 1.36 M
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