Concentration

Concentration describes how much solute is dissolved in a given amount of solvent or solution. There are several ways to express concentration, each useful in different contexts.

Expressing Concentration

Percentage by Mass (Mass/Mass) - m/m %

This expresses the ratio of the mass of solute to the total mass of the solution.

Where:

• Mass of solute = mass of dissolved substance
• Mass of solution = mass of solute + mass of solvent
• Result is expressed as a percentage (%)

Ejemplo: A 5% salt solution contains 5 g of salt dissolved in 95 g of water (total solution = 100 g)

We apply the formula: \(\frac{5 \text{ g}}{100 \text{ g}} \times 100 = 5\%\)

Ejemplo: Calculate the percentage by mass of 12 g of sugar dissolved in 288 g of water

Calculate the mass of solution: Mass of solution = 12 g + 288 g = 300 g Apply the formula: \(\frac{12 \text{ g}}{300 \text{ g}} \times 100 = 4\%\)

Advantages:

• Independent of temperature (mass doesn't change with temperature)
• Easy to prepare in the laboratory
• Used in industrial applications where mass is easier to measure

Common applications: Food industry, pharmaceutical preparations, disinfectant solutions

Percentage by Volume (Volume/Volume) - v/v %

This expresses the ratio of the volume of solute to the total volume of the solution.

Where:

• Volume of solute = volume of dissolved substance
• Volume of solution ≈ volume of solute + volume of solvent (assuming volumes are additive)
• Result is expressed as a percentage (%)

Ejemplo: A 10% alcohol solution contains 10 mL of alcohol in 100 mL of solution

We apply the formula: \(\frac{10 \text{ mL}}{100 \text{ mL}} \times 100 = 10\%\)

Ejemplo: Calculate the percentage by volume of 25 mL of ethanol mixed with 75 mL of water

Calculate the volume of solution: Volume of solution = 25 mL + 75 mL = 100 mL Apply the formula: \(\frac{25 \text{ mL}}{100 \text{ mL}} \times 100 = 25\%\)

Advantages:

• Used for liquid-liquid solutions
• Practical for solutions where volumes are easy to measure
• Important in laboratory work and beverage preparation

Common applications: Alcoholic beverages, disinfectant sprays, vinegar solutions, rubbing alcohol

Mass/Volume (m/v) - Alternative Unit

This expresses the ratio of the mass of solute to the volume of the solution. While less common than percentage by mass or volume, it's sometimes used.

Ejemplo: A solution containing 10 g of salt dissolved in water to make 500 mL of solution

Calculate the m/v concentration: \(\frac{10 \text{ g}}{500 \text{ mL}} = 0.02 \text{ g/mL}\) or \(20 \text{ g/L}\)

Advantages:

• Combines advantages of both mass and volume measurements
• Useful when solute is solid but measured volume is important

Common applications: Some pharmaceutical solutions, laboratory reagents

Comparison of the Three Methods

Key Difference:

• m/m uses mass of both solute and solution
• v/v uses volume of both solute and solution
• m/v uses mass of solute but volume of solution