What are the different units of strength of solution?

Several units are used to express the strength or concentration of a solution. Here's a breakdown of the most common ones:

Molarity (M)

Molarity is defined as the number of moles of solute per liter of solution.

• Formula: Molarity (M) = Moles of solute / Liters of solution
• Units: mol/L or M
• Example: A 1 M solution of NaCl contains 1 mole of NaCl dissolved in 1 liter of solution.

Molality (m)

Molality is defined as the number of moles of solute per kilogram of solvent.

• Formula: Molality (m) = Moles of solute / Kilograms of solvent
• Units: mol/kg or m
• Example: A 1 m solution of glucose contains 1 mole of glucose dissolved in 1 kilogram of water.

Normality (N)

Normality is defined as the number of gram equivalent weights of solute per liter of solution. The "equivalent weight" depends on the reaction the compound undergoes.

• Formula: Normality (N) = Gram equivalent weights of solute / Liters of solution
• Units: eq/L or N
• Example: A 1 N solution of H₂SO₄ will contain 49 grams of H₂SO₄ (molecular weight 98 g/mol, equivalent weight 49 g/eq because it can donate 2 protons) per liter of solution. Normality is particularly useful in acid-base titrations.

Mass Percentage (%)

Mass percentage expresses the concentration of a solution as the mass of the solute divided by the total mass of the solution, multiplied by 100.

• Formula: Mass % = (Mass of solute / Mass of solution) x 100
• Units: %
• Example: A 10% by mass NaCl solution contains 10 grams of NaCl in 100 grams of solution.

Volume Percentage (%)

Volume percentage expresses the concentration of a solution as the volume of the solute divided by the total volume of the solution, multiplied by 100. This is usually used when both the solute and solvent are liquids.

• Formula: Volume % = (Volume of solute / Volume of solution) x 100
• Units: %
• Example: A 5% by volume ethanol solution contains 5 mL of ethanol in 100 mL of solution.

Parts per Million (ppm) and Parts per Billion (ppb)

These units are used for very dilute solutions, expressing the concentration as the number of parts of solute per million or billion parts of solution.

• Formulas:
  • ppm = (Mass of solute / Mass of solution) x 10⁶
  • ppb = (Mass of solute / Mass of solution) x 10⁹
• Units: ppm or ppb
• Example: A water sample containing 2 ppm of lead contains 2 mg of lead per kg of water (approximately 1 L of water).

Mole Fraction (χ)

Mole fraction is defined as the number of moles of a component divided by the total number of moles of all components in the solution.

• Formula: Mole fraction of solute (χ_solute) = Moles of solute / (Moles of solute + Moles of solvent)
• Units: Dimensionless (unitless)
• Example: In a solution containing 1 mole of ethanol and 9 moles of water, the mole fraction of ethanol is 1 / (1+9) = 0.1.

In summary, the choice of unit depends on the specific application and the desired level of accuracy. Molarity and molality are common in chemistry, while ppm and ppb are often used in environmental science. Normality is valuable for titrations.