Molarity
- Molarity (M) can be defined as the ratio of number of moles of solute to the volume of solution in liter.
- It can also be defined as the number of moles of solute present in unit litre or cubic decimeter of solution.
- It is function of temperature due to the dependence of volume on temperature whereas Mass %, ppm, mole fraction and molality are independent of temperature because mass does not depend on temperature.
- Mathematically, Molarity = Moles of Solute / Volume of Solution in Litre
- For instance, molarity of a solution containing 0.25 mol of NaOH dissolved in one litre or one cubic decimeter is 0.25 M or 0.25 mol L–1.
Problem: Calculate the molarity of each of the following solutions:
(a) 30 g of Co (NO3)2. 6H2O in 4.3 L of solution
(b) 30 mL of 0.5 M H2SO4 diluted to 500 mL.
Solution:
Molarity = Moles of Solute / Volume of Solution in Litre
(a) Molar mass of Co (NO3)2.6H2O (solute)
= 59 + 2 (14 + 3 × 16) + 6 × 18 = 291 g mol - 1
∴Moles of Co(NO3)2.6H2O = 30 / 291 mol = 0.103 mol
Therefore, molarity = 0.103 mol / 4.3 L = 0.023 M
(b) Number of moles present in 1000 mL (Solvent) of 0.5 M H2SO4 (Solute) = 0.5 mol
∴ Number of moles present in 30 mL of 0.5 M H2SO4 = (0.5 X 30 ) / 1000 mol = 0.015 mol
Therefore, molarity = 0.015 mol / 0.5 L = 0.03 M
