Molarity: The Solution’s Strength

Molarity is a unit of concentration in chemistry. Molarity is typically used to measure the concentration of a solute in a solution. It is an important concept, as it is used to calculate the amount of a given substance, present in a solution.

Concentration measures how much of a substance is dissolved in a given amount of solvent. There are several units used to express concentration, including molarity, molality, normality, etc. Molarity, however, is one of the most commonly used and important units of concentration.

Molarity

The number of moles of solute dissolved per liter of the solution is called the molarity of the solution.

Formula

M = moles of solute/liters of solution

M = n/ (V in L)

M = (moles/molar mass)(1/liters of solution)

M = (n/M₁)(1/V in L)

Units

The molarity of one-mole solute dissolved in one one-liter solution is one molar (1M).

mol/L or mol/dm³ or M

(1 L  = 1 dm³ )

The SI unit is, however;

mol/m³

Example no. 1

20 grams of NaOH is dissolved in one liter of solution. Calculate the molarity of the solution.

Solution:

Mass of solute ( NaOH) = 20 g

Molar mass of NaOH = 40 g/mol

Volume of solution = 1  L

M =?

M = (n/M₁)(1/V)

= (20/40)(1/1)

Molarity = 0.5 M

Importance of Molarity

Molarity is the concentration unit and it is used to express the number of moles of solute dissolved per liter of the solution. Molarity is concerned with the volume of the solution. Therefore it is dependent upon pressure and temperature as volume changes with a change in these parameters.

Molarity is in greater use compared to normality because it is easier to calculate molar weight than equivalence weight.

Relation with Molality

M = (1000ρm)/(1000+mM₁)

where,

• M = Molarity
• ρ = Density of solution (g/cm³)
• m = Molality of solution
• M₁ = Molar mass of solute

Example no. 2

Calculate the molarity of 2.09 molal HCl solution having a density of 1.03 g/cm³.

Given:

Molality = m = 2.09 m

Density = ρ = 1.03 g/cm³ 

Molar mass of HCl = M₁ = 36.5 g/mol

Solution:

M =?

M = (1000ρm)/(1000+mM₁)

M = 1000(1.03)(2.09)/1000+(2.09)(36.5)

Molarity = 2 M

Example no. 3

Calculate the molarity of 3.17 molal NaCl solution with a density of 1.12 g/cm³.

Given:

Molality = m = 3.17 m

Density = ρ = 1.12 g/cm³ 

Molar mass of NaCl = M₁ = 58.5 g/mol

Solution:

M = (1000ρm)/(1000+mM₁)

M = 1000(1.12)(3.17)/1000+(3.17)(58.5)

Molarity = 3 M

Related content

• Molality: The Solution’s Signature
• Atomic Mass vs. Molar Mass: The Basic Concepts of Chemistry
• Molarity vs. Molality: Which One to Prefer?

Molarity vs. Molality

Dilution Formula of Molarity

The dilution formula is used to make the solution of desired molarity from the stock solution.

M₁V₁ = M₂V₂

where,

• M₁ = Molarity of stock solution
• V₁ = Volume used from the stock solution
• M₂ = Desired molarity
• V₂ = Desired volume

Molarity from Percentage Purity 

Molarity = (%purity)(density)(10)/molar mass

Example no. 4

The density of 98%-pure sulphuric acid is 1.84 g/mL. Calculate molarity.

Given:

Density = 1.84 g/mL

%purity = 98 %

Molar mass of H₂SO₄ = 98 g/mol

Solution:

M = (%purity)(density)(10)/molar mass

M = (98)(1.84)(10)/98

Molarity = 18.4 M

Molarity and Normality

Normality = M × basicity

or

Normality = M × acidity

N = M × n

where,

• n= Basicity or Acidity

If n= 1 then,

N = M

The basicity of an acid is the number of H⁺ produced by an acid in an aqueous solution. e.g basicity of HCl is 1 and H₂SO₄ is 2.

The acidity of a base is the number of OH^– produced by a base in an aqueous solution. e.g acidity of NaOH is 1 and Ba(OH)₂ is 2.

Strength = M × mol. wt = N × eq.wt

Example no. 5

Calculate the normality of the 18.4 M sulphuric acid solution.

Solution:

M = 18.4

n for H₂SO₄ = 2

N = M × n

N = 18.4 (2)

Normality = 36.8 N

Concepts Berg

How do I calculate the normality of an acid or base from its molarity?

N = M × n

∴ (n = Basicity or Acidity)

For example, the normality of 18.4 M H₂SO₄;

M = 18.4

n for H₂SO₄ = 2

N = M × n

N = 18.4  × 2

Normality = 36.8 N

What is the equation used for molarity conversion?

The dilution formula is used for conversion.

M₁V₁ = M₂V₂

How many molecules can be found in a reaction?

This depends on the type of reaction and the reactants involved. Generally speaking, a reaction involves two or more reactants, which can form one or more products. Therefore, the number of molecules in a reaction can range from two to potentially millions.

Is solubility = molarity = molality? 

No, solubility, molarity, and molality are different measurements. Solubility is the maximum amount of a substance dissolved in a given solvent. Molarity is the number of moles of a solute per liter of a solution, and molality is the number of moles of a solute per kilogram of a solvent.

How do you make a 0.1 M solution? 

To make a 0.1 M solution, you must dissolve 0.1 moles of the solute in enough solvent to make one liter of solution. For example, if your solute is sodium chloride, you would weigh out 0.1 moles of sodium chloride (5.85 g) and dissolve it in 1 liter of distilled water.

What units are of molarity? 

mol/L or mol/dm³ or M

The molarity of one-mole solute dissolved in one one-liter solution is one molar (1M).

The SI unit is mol/m³.

What is the basic formula of molarity?

M = moles of solute/liters of solution

References Books

• Engineering Chemistry By A.K. Pahari, B.S. Chauhan
• General, Organic, and Biological Chemistry By H. Stephen Stoker

Reference links

• Differences: Formula and Definitions (technologynetworks.com)