Solutions
Section: 12. Experimental Techniques & Chemical Analysis | Syllabus: Cambridge AS Level Physics 9702
What is a Solution? Solution A mixture formed when a solute dissolves in a solvent to form a homogeneous mixture. Solute The substance that dissolves (usually a solid, but can be liquid or gas). Solvent The liquid in which the solute dissolves (usually water - the "universal solvent").
Relationship: Solute + Solvent → Solution Example: Sodium chloride (solute) + Water (solvent) → Sodium chloride solution Concentration of Solutions Concentration The amount of solute dissolved in a given volume of solution.
It tells us how "strong" or "weak" a solution is. Ways to Express Concentration: Method Units Formula When Used Grams per dm³ g/dm³ concentration = mass of solute (g) / volume of solution (dm³) Simple calculations, non-ionic substances Moles per dm³ (Molarity) mol/dm³ or M concentration = moles of solute / volume of solution (dm³) Chemical reactions, titrations Unit Conversion 1 dm³ = 1000 cm³ = 1 litre To convert cm³ to dm³: divide by 1000 Example: 250 cm³ = 250 ÷ 1000 = 0.25 dm³ Calculating Concentration in g/dm³ Formula: Concentration (g/dm³) = Mass of solute (g) / Volume of solution (dm³) Example 1: Calculate the concentration when 20 g of sodium chloride is dissolved in 500 cm³ of water.
Solution: Mass of solute = 20 g Volume = 500 cm³ = 500 ÷ 1000 = 0.5 dm³ Concentration = 20 g ÷ 0.5 dm³ = 40 g/dm³ Example 2: What mass of glucose is needed to make 250 cm³ of a solution with concentration 80 g/dm³?
Solution: Volume = 250 cm³ = 0.25 dm³ Rearrange: Mass = Concentration × Volume Mass = 80 g/dm³ × 0.25 dm³ = 20 g Calculating Concentration in mol/dm³ Formula: Concentration (mol/dm³) = Moles of solute / Volume of solution (dm³) Remember: Moles = Mass (g) ÷ Molar mass (g/mol) Example 3: Calculate the concentration in mol/dm³ when 11.7 g of sodium chloride (NaCl) is dissolved in 500 cm³ of water.
(Mr of NaCl = 58.5) Solution: Step 1: Calculate moles of NaCl Moles = 11.7 g ÷ 58.5 g/mol = 0.2 mol Step 2: Convert volume to dm³ Volume = 500 cm³ = 0.5 dm³ Step 3: Calculate concentration Concentration = 0.2 mol ÷ 0.5 dm³ = 0.4 mol/dm³ Dilute vs Concentrated Solutions Type Description Example Dilute Solution Contains a small amount of solute relative to solvent Low concentration 0.1 mol/dm³ hydrochloric acid Concentrated Solution Contains a large amount of solute relative to solvent High concentration 12 mol/dm³ hydrochloric acid Saturated Solution Contains the maximum amount of solute that can dissolve at that temperature No more solute can dissolve Salt solution with undissolved crystals at bottom Important Distinction Concentrated ≠ Strong and Dilute ≠ Weak • Concentration refers to amount of substance present • Strength (for acids/bases) refers to degree of ionization You can have a dilute solution of a strong acid (e.g., 0.1 M HCl) Preparing a Standard Solution Standard Solution A solution of accurately known concentration, used in titrations and quantitative analysis.
Method: Calculate the mass of solute needed using the formula Weigh the calculated mass accurately using a balance Dissolve the solute in a small volume of distilled water in a beaker Transfer the solution to a volumetric flask using a funnel Rinse the beaker and funnel with distilled water, adding rinsings to flask Add distilled water to the flask until near the calibration mark Make up to the mark using a dropper, adding water dropwise Stopper the flask and invert several times to mix thoroughly Diagram Step-by-step illustration showing: (1) Weighing boat on balance with solid.
(2) Beaker with solid being dissolved in small volume of water, stirring rod. (3) Funnel in volumetric flask, solution being poured in. (4) Rinsing beaker with wash bottle into funnel. (5) Adding water to flask up to graduation mark using dropper.
(6) Stoppered flask being inverted to mix. Label each step 1-6 and add key notes: "Dissolve completely", "Transfer all solution", "Rinse thoroughly", "Make up to mark carefully", "Mix well". Key Points Use a volumetric flask for accurate volume Rinse all apparatus to ensure complete transfer Add final drops of water with a dropper for precision Read the meniscus at eye level Mix thoroughly by inverting the stoppered flask Diluting Solutions When diluting a concentrated solution, the number of moles stays the same, but the volume increases: Formula: C₁V₁ = C₂V₂ Where: C₁ = initial concentration, V₁ = initial volume C₂ = final concentration, V₂ = final volume Example: How would you prepare 250 cm³ of 0.1 mol/dm³ HCl from 2.0 mol/dm³ HCl?
Solution: C₁ = 2.0 mol/dm³, C₂ = 0.1 mol/dm³, V₂ = 250 cm³ C₁V₁ = C₂V₂ 2.0 × V₁ = 0.1 × 250 V₁ = 25 ÷ 2.0 = 12.5 cm³ Method: Measure 12.5 cm³ of concentrated acid with a pipette Add to volumetric flask and make up to 250 cm³ with distilled water Safety - Diluting Acids Always add acid to water, NEVER water to acid!
Adding water to concentrated acid can cause violent boiling and splashing. Remember: "Do as you oughta, add acid to water!" Summary: Solutions Solution = Solute dissolved in solvent…
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