Concentration of Aqueous Solutions
Definition of Concentration: Concentration refers to how much solute (the substance being dissolved) is present in a certain amount of solution. It tells us how “strong” or “weak” a solution is depending on how much solute it contains.
Units: Grams per Cubic Decimetre (g dm⁻³): This unit measures how many grams of solute are dissolved in one cubic decimetre (which is the same as one litre) of solution. It helps us understand the mass of solute in a given volume.
Units: Moles per Cubic Decimetre (mol dm⁻³): This is also known as molarity (M). It measures how many moles of solute are present in one cubic decimetre of solution. A mole is a way of counting particles like atoms or molecules.
Calculation of Concentration (g dm⁻³): To find the concentration in grams per cubic decimetre, you simply divide the mass of the solute (in grams) by the volume of the solution (in cubic decimetres).
Calculation of Molarity (mol dm⁻³): To find the molarity, divide the number of moles of solute by the volume of the solution in cubic decimetres. This gives you the molar concentration.
Relationship Between Concentration and Molarity: There is a direct relationship between the mass concentration and molarity: Concentration (g dm⁻³) = Molarity (mol dm⁻³) × Molar Mass (g mol⁻¹). This formula connects mass, moles, and volume.
Standard Solutions
Definition of Standard Solutions: A standard solution is a solution where the exact concentration is known. Chemists use standard solutions to perform accurate experiments and chemical analyses.
Preparation Using Volumetric Flasks: Volumetric flasks are special pieces of equipment designed to make precise volumes of solution. They are used to ensure the solution has the correct concentration.
Direct Preparation Method: This method involves carefully weighing a specific mass of solute, dissolving it in some solvent, and then adding more solvent until the total volume reaches a marked line on the volumetric flask.
Dilution from a Stock Solution: Sometimes, instead of making a solution from scratch, chemists take a more concentrated solution called a stock solution and add solvent to it. This lowers the concentration to the desired level.
Dilution Calculation Formula: To calculate how much stock solution to use, we use the formula M₁V₁ = M₂V₂, where M₁ and V₁ are the molarity and volume of the stock solution, and M₂ and V₂ are the molarity and volume of the diluted solution.