Solved Examples on Molarity

Example 1: A solution was prepared using 20 g of sodium hydroxide. Calculate the molarity of the given solution of sodium hydroxide if the volume of the solution is 125 ml.

Solution:

Mass of sodium hydroxide (NaOH) in the given solution = 20 g

The volume of the solution = 125 ml = 125/1000 = 0.125 L

The molar mass of sodium hydroxide = 40g/mol

Number of moles of sodium hydroxide (n) = Mass of NaOH/Molar mass of NaOH

= 20/40 = 0.5 moles

We know that,

Molarity (M) = Moles of NaOH/Volume of the solution

M = 0.5/0.125 = 4

Hence, the molarity of the solution is 4M.

Example 2: A solution is prepared by mixing 10 grams of ethanol. Calculate the molarity of the given solution if the volume of the solution is 25.6 ml.

Solution:

The volume of the solution (V) = 25.6 ml = 25.6/1000 = 0.0256 L

Mass of ethanol in the given solution= 10 g

The molecular weight of ethanol = 46.07 g/mol

Number of moles of ethanol (n) = 10 g/46.07 g/mol

= 0.217 moles

We know that,

Molarity (M) = n/V

M = 0.217/0.0256 = 8.477 M

Hence, the molarity of the solution is 8.477M.

Example 3: A solution is prepared by bubbling 1.825 grams of hydrochloric acid in water. Calculate the molarity of the given solution if the volume of the solution is 12.8 ml.

Solution:

The volume of the solution (V) = 12.8 ml = 12.8/1000 = 0.0128 L

The mass of HCl in the given solution = 1.825 g

The molecular weight of HCl = 36.5 g/mol

Number of moles of hydrochloric acid (n) = 7.3 g/36.5 g/mol

= 0.05 moles

We know that,

Molarity (M) = n/V

M = 0.05/0.0128 = 3.91 M

Hence, the molarity of the solution is 3.91M.

Example 4: A solution is prepared by mixing 10 grams of calcium chloride. Calculate the molarity of the calcium chloride solution if the volume of the solution is 200 ml.

Solution:

The mass of CaCl₂ in the given solution = 10 g

The molecular weight of CaCl₂ = 110.98 g/mol

Number of moles of CaCl₂ = 10 g/ (110.98 g/mol)

= 0.0901 moles

The volume of the solution (V) = 200 ml = 200/1000 = 0.2 L

We know that,

Molarity (M) = n/V

M = 0.0901/0.2 = 0.4505 M

Hence, the molarity of the solution is 0.451 M.

Example 5: We have 25 mL of a 2.56 M carbon tetrachloride solution, but we need a 1.28 M solution. So, how many mL of the 1.28M solutions can we prepare?

Solution:

Initial Molarity of the solution (M1) = 2.56 M

The initial volume of the given solution (V1) = 25 mL

Molarity of the new solution (M2) = 2.56 M

The volume of the new solution (V2) =?

We know that,

M₁V₁ = M₂V₂

⇒ V₂ = M₁V₁/M₂

⇒ V₂ = (2.56 × 25/1000)/1.28

= 50/1000 = 50 mL

Hence, the volume of the new solution is 50 mL.

Molarity is defined as total number of moles of solute per litre of solution. It is represented by M. In chemistry, the molarity is one of the concentration units. The concentration of solutions can be expressed more effectively using measurements based on moles. The SI unit of Molarity is Mole.

A mole is an SI unit that measures the amount of a substance, where the amount of a substance is a measurement of the number of its elementary entities present in an object or sample. According to the definition, a mole contains exactly 6.022 × 10²³ elementary entities, where 6.022 × 10²³ is called Avogadro’s number (N_A). Molarity is used to measure the concentration of a solution, where a concentrated solution is a solution with more solute and less solvent.