Molarity o Molarity( M) can be defined as the ratio of number of moles of solute to the volume of solution in liter. o
Molarity o Molarity( M) can be defined as the ratio of number of moles of solute to the volume of solution in liter. o
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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