In analytical chemistry, expressing concentration and content accurately is critical for proper interpretation of results. Below is a detailed outline of different types of concentration expressions:
A. Different Concentrations and How They Are Expressed
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Molarity (M): Moles of solute per liter of solution.
- Formula: M = moles of solute / liters of solution
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Molality (m): Moles of solute per kilogram of solvent.
- Formula: m = moles of solute / kg of solvent
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Weight/Volume (w/v): Grams of solute per 100 mL of solution.
- Formula: w/v = grams of solute / 100 mL solution
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Weight/Weight (w/w): Grams of solute per 100 grams of solution.
- Formula: w/w = grams of solute / 100 g solution
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Volume/Volume (v/v): Milliliters of solute per 100 mL of solution.
- Formula: v/v = mL of solute / 100 mL solution
B. Percent Weight by Weight (w/w)
Definition: Percent weight by weight is the mass of solute divided by the total mass of the solution, multiplied by 100.
- Formula: % w/w = (mass of solute / mass of solution) × 100
Example: If you have 5 grams of sodium chloride dissolved in 95 grams of water, the total mass of the solution is 100 grams.
- % w/w of NaCl = (5 g / 100 g) × 100 = 5% w/w
C. Percent Volume by Volume (v/v)
Definition: Percent volume by volume is the volume of solute divided by the total volume of the solution, multiplied by 100.
- Formula: % v/v = (volume of solute / volume of solution) × 100
Example: If you have 30 mL of ethanol in 70 mL of water, the total volume of the solution is 100 mL.
- % v/v of ethanol = (30 mL / 100 mL) × 100 = 30% v/v
D. Parts per Thousand (ppt) and Parts per Million (ppm)
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Parts per Thousand (ppt): This unit describes the concentration of a substance in a solution, with reference to a base of 1,000.
- Formula: 1 ppt = 1 g of solute per 1,000 g of solution or 1 mg per 1,000 mL
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Parts per Million (ppm): Describes the concentration of one part of substance in one million parts of the solution.
- Formula: 1 ppm = 1 mg of solute per 1,000,000 mL of solution or 1 g of solute per 1,000,000 g of solution
Example: If a water sample has 0.01 mg of mercury in 1 L (1,000 mL):
- Concentration = (0.01 mg / 1,000,000 mg) × 1,000,000 ppm = 10 ppm
E. Formality and Its Relation to Molality and Molarity
Formality (F): Molar concentration of a substance in a solution based on the chemical formula, regardless of the solubility or species present. It considers the total species present from a compound in solution.
- Formula: F = moles of solute / liters of solution
Relation:
- Formality can be similar to molarity when the solution is clear and all the solute is in a single molecular form.
- Molality considers the mass of the solvent, while molarity considers the volume of the solution. In cases where temperature and pressure can affect the volume of the solution, formality can provide a more consistent measurement of concentration since it relies on mole counts not affected by solute volume changes.
F. Equivalent Weights and Normality
- Equivalent Weight: The amount of a substance that can react with or provide one mole of hydrogen ions (H⁺) or one mole of electrons in a redox reaction.
- Formula: Equivalent weight = Molar mass / n (where n is the number of particles the compound can donate or accept)
- Normality (N): The number of equivalents of solute per liter of solution.
- Formula: N = equivalents of solute / liters of solution
Example: For sulfuric acid (H₂SO₄), which can donate 2 protons (H⁺), if its molar mass is ~98 g/mol:
- Equivalent weight of H₂SO₄ = 98 g/mol / 2 = 49 g/equiv
- If you prepare a solution containing 49 g of H₂SO₄ in 1 liter, the normality would be 1 N because it can donate 2 protons per mole, meaning it provides 2 equivalents.
This completes the overview of concentration expressions in analytical chemistry. Each type serves a specific purpose depending on the chemical context and the needs of the analysis.