You can measure the concentration of a solution by determining the amount of solute present in a known amount of solvent or solution. There are several methods, each expressing concentration in different units. The fundamental principle involves quantifying the solute and relating it to the overall solution volume or mass.
Here's a breakdown of common methods:
- Molarity (M): This is one of the most frequently used methods.
- Definition: Molarity is defined as the number of moles of solute per liter of solution.
- Formula: Molarity (M) = Moles of solute / Liters of solution
- Example: If you dissolve 1 mole of NaCl in enough water to make 1 liter of solution, the solution is 1 M NaCl.
- Molality (m): Especially useful when temperature changes affect the volume of the solution.
- Definition: Molality is defined as the number of moles of solute per kilogram of solvent.
- Formula: Molality (m) = Moles of solute / Kilograms of solvent
- Example: If you dissolve 1 mole of NaCl in 1 kg of water, the solution is 1 m NaCl.
- Percent Composition (by mass): Expresses the mass of the solute as a percentage of the total mass of the solution.
- Definition: (Mass of solute / Mass of solution) x 100%
- Formula: % Composition = (Mass of solute / (Mass of solute + Mass of solvent)) x 100%
- Example: If you dissolve 10g of sugar in 90g of water, the percent composition by mass is (10g / 100g) x 100% = 10%.
- Volume Percent (% v/v): Expresses the volume of the solute as a percentage of the total volume of the solution. This is often used when dealing with liquid solutions.
- Definition: (Volume of solute / Volume of solution) x 100%
- Formula: % Volume = (Volume of solute / Volume of solution) x 100%
- Example: If you mix 20 mL of ethanol with enough water to make 100 mL of solution, the volume percent is (20 mL / 100 mL) x 100% = 20%.
- Parts per Million (ppm) and Parts per Billion (ppb): Used for very dilute solutions.
- Definition: Expresses the amount of solute as parts per million or billion parts of solution, by mass or volume.
- Formulas:
- ppm = (Mass of solute / Mass of solution) x 106 or (Volume of solute / Volume of solution) x 106
- ppb = (Mass of solute / Mass of solution) x 109 or (Volume of solute / Volume of solution) x 109
- Example: Finding trace contaminants in water.
- Normality (N): Used in acid-base chemistry, related to molarity, but expresses concentration in terms of equivalents.
- Definition: Normality is defined as the number of gram equivalent weights of solute per liter of solution.
- Formula: Normality (N) = Gram equivalent weights of solute / Liters of solution
- Example: A 1 N H2SO4 solution is 0.5 M H2SO4 because sulfuric acid has two acidic protons.
Here's a table summarizing the common concentration units:
| Concentration Unit | Definition | Formula |
|---|---|---|
| Molarity (M) | Moles of solute per liter of solution | M = Moles of solute / Liters of solution |
| Molality (m) | Moles of solute per kilogram of solvent | m = Moles of solute / Kilograms of solvent |
| % Composition | (Mass of solute / Mass of solution) x 100% | % Composition = (Mass of solute / Mass of solution) x 100% |
| % Volume | (Volume of solute / Volume of solution) x 100% | % Volume = (Volume of solute / Volume of solution) x 100% |
| ppm | Parts per million | ppm = (Mass of solute / Mass of solution) x 106 |
| ppb | Parts per billion | ppb = (Mass of solute / Mass of solution) x 109 |
| Normality (N) | Gram equivalent weights of solute per liter of solution | N = Gram equivalent weights of solute / Liters of solution |
In essence, measuring the concentration of a solution involves quantifying the amount of solute in a defined quantity of solvent or the solution itself.
