The Most Important Concentration Units like Molarity, Molality, Normality, Formality etc with numerical problems Class 12th Solutions chapter.
Concentration Units with Numericals
1. Molality (m)
Molality (m) is defined as the number of moles of solute per kilogram of solvent.
Formula: Molality (m) = moles of solute / mass of solvent (kg)
Example:
Calculate the molality of a solution containing 0.5 moles of NaCl dissolved in 2 kg of water.
Solution: Molality (m) = 0.5 moles / 2 kg = 0.25 m
2. Molarity (M)
Molarity (M) is the number of moles of solute per liter of solution.
Formula: Molarity (M) = moles of solute / volume of solution (L)
Example:
Find the molarity of a solution containing 0.75 moles of H2SO4 in 1.5 liters of solution.
Solution: Molarity (M) = 0.75 moles / 1.5 L = 0.5 M
3. Normality (N)
Normality (N) is defined as the number of gram equivalents of solute per liter of solution.
Formula: Normality (N) = gram equivalents of solute / volume of solution (L)
Example:
Calculate the normality of a solution containing 49 grams of H2SO4 (molar mass = 98 g/mol) in 1 liter of solution.
Solution: Gram equivalents of solute = 49 g / 49 g/equiv. = 1 equiv.
Normality (N) = 1 equiv. / 1 L = 1 N
4. Formality (F)
Formality (F) is similar to molarity but is used when the solute does not fully dissolve in a solution, especially for ionic compounds.
Formula: Formality (F) = moles of solute / volume of solution (L)
Example:
Find the formality of a solution with 3 moles of NaCl in 1.5 liters of solution.
Solution: Formality (F) = 3 moles / 1.5 L = 2 F
5. Mole Fraction (x)
Mole fraction (x) is the ratio of the number of moles of one component to the total number of moles of all components in the solution.
Formula: Mole Fraction (x) = moles of component / total moles of all components
Example:
In a solution with 2 moles of ethanol (C2H5OH) and 3 moles of water, calculate the mole fraction of ethanol.
Solution: Mole fraction of ethanol = 2 / 2 + 3 = 0.4
6. Mass Percentage (% w/w)
Mass percentage is the mass of the solute divided by the total mass of the solution, multiplied by 100.
Formula: % w/w = mass of solute / total mass of solution × 100
Example:
Find the mass percentage of a solution containing 20 g of NaOH dissolved in 80 g of water.
Solution: % w/w = 20 g / 20 g + 80 g × 100 = 20%
7. Volume Percentage (% v/v)
Volume percentage is the volume of the solute divided by the total volume of the solution, multiplied by 100.
Formula: % v/v = volume of solute / total volume of solution × 100
Example:
Calculate the volume percentage of ethanol if 50 mL of ethanol is mixed with 150 mL of water.
Solution: % v/v = 50 mL / 50 mL + 150 mL × 100 = 25%
8. Mass by Volume Percentage (% w/v)
This unit expresses the mass of solute per volume of solution, multiplied by 100.
Formula: % w/v = mass of solute / volume of solution × 100
Example:
Find the mass by volume percentage of a solution containing 5 g of sugar in 250 mL of water.
Solution: % w/v = 5 g / 250 mL × 100 = 2%
