The Fundamental Unit of Life | Class 9 Science Notes

5.2.2 How do we express concentration?

Concentration of solutions can be expressed in several ways, with the percentage method being common. The three main types are:

A. Mass by mass percentage (% m/m or % w/w): This expresses how many grams of solute are present in 100 grams of the total solution. It is used for homogeneous mixtures and some heterogeneous mixtures like milk powder and spice mixtures. The formula is: Mass by mass percentage = (Mass of solute / Mass of solution) × 100

Example: If 10 g of salt is dissolved in 90 g of water, the total solution mass is 100 g. The mass by mass percentage of salt is (10/100) × 100 = 10% m/m.

B. Mass by volume percentage (% m/v or % w/v): Used when measuring volume is easier than mass, common in medicines and labs. It tells how many grams of solute are present in 100 mL of solution. The formula is: Mass by volume percentage = (Mass of solute / Volume of solution) × 100

Example: If 5 g of glucose is dissolved to make 100 mL of solution, concentration is (5/100) × 100 = 5% m/v.

C. Volume by volume percentage (% v/v): Used when two miscible liquids are mixed, such as in perfumes and vinegar. It tells how many mL of solute are present in 100 mL of solution. The formula is: Volume by volume percentage = (Volume of solute / Volume of solution) × 100

Example: Mixing 1 mL of liquid pesticide in 100 mL of solution gives 1% v/v.

These methods help in labeling commercial products and preparing solutions accurately for various applications.

📊 Diagram: Fig. 5.4: Composition of some commercial packaged products (a) milk powder, (b) glucose solution, and (c) vinegar; Fig. 5.5: Saline solution

🔗 Connection: This section connects to the next section on solubility, explaining how much solute can dissolve in a solvent.