Volumetric Analysis Notes PDF
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Ms. Christine Gatt
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These notes provide a detailed explanation of volumetric analysis techniques, focusing on preparing standard solutions and performing titrations. The examples cover calculations for concentrations and amounts of substances, offering a step-by-step guide for chemical experiments.
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TOPIC: VOLUMETRIC ANALYSIS LO 7: Identification of soluble substances and measurement of their concentration. By the end of this topic, I should be able to: ✓ Perform percolations involving moles and molar concentrations. (7.3g) ✓ prepare a standard solution using step by ste...
TOPIC: VOLUMETRIC ANALYSIS LO 7: Identification of soluble substances and measurement of their concentration. By the end of this topic, I should be able to: ✓ Perform percolations involving moles and molar concentrations. (7.3g) ✓ prepare a standard solution using step by step instructions. (Limited to sodium carbonate) (7.2h, 7.3h) ✓ Conduct an acid/base titration using step by step instructions. (7.2i) ✓ Conduct an acid/base titration to determine the concentration of a given solution. (Example: hydrochloric acid/sulfuric acid/nitric acid/ethanoic acid with sodium hydroxide/potassium hydroxide/sodium carbonate.) (7.3i) ✓ Calculate the concentration/volume of a solution taking part in a reaction. (7.3j) Volumetric analysis is a process used to determine the amount of the desired substance by its volume. This is a branch of quantitative analysis. By means of volumetric analysis we can determine the concentration of a solution. The procedure used is known as a titration. Titrations are commonly used when carrying out acid- base reactions. CONCENTRATION The concentration of a solute in a solution is the number of moles of solute present in 1 Litre of solution. Note: 1 Litre = 1000 cm3 = 1 dm3 Concentration (Moles/L) or Molarity (M) = Amount of solute (Moles) Volume of solution (Litres) Remember: Molarity = Moles Volume If 2 moles of solute is present in 1 L of solution, the concentration of solute is: 2mole/L OR 2mole/dm3 OR 2M (Molar) If 4 moles of solute is present in 10 L of solution, the concentration of solute is: 0.4mole/L OR 0.4mole/dm3 OR 0.4M (Molar) Ms. Christine Gatt Page 1 of 10 Worked examples: First, write the balanced chemical equation for the reaction: a. Ms. Christine Gatt Page 2 of 10 b. c. Ms. Christine Gatt Page 3 of 10 STANDARD SOLUTIONS A standard solution is one which has been made up with a known concentration of solute using a very pure sample of distilled water. Laboratory Preparation of a Standard Solution Precautions: 1) Clean the watch-glass and spatula before use. 2) Wash the last traces of solute from the watch-glass and into the beaker. (For quantitative transfer) Ms. Christine Gatt Page 4 of 10 Not all compounds are suitable to prepare standard solutions. Compounds absorb water vapour from air, so their mass varies. Example: sodium hydroxide is deliquescent, concentrated sulfuric acid is hygroscopic. Some alkaline compounds react with carbon dioxide in air. Example: sodium hydroxide And potassium hydroxide. Some compounds evaporate easily (volatile). Example: Concentrated hydrochloric acid and ammonia. Standard solutions for such compounds cannot be prepared with the procedure outlined previously. Worked Example: What mass of sodium carbonate is needed to prepare 250.0 mL of a 0.05 mol/L solution of sodium carbonate? Step 1: Calculate the number of moles of sodium carbonate needed. Concentration in mol/dm3 = moles of solute volume in dm3 moles of solute = concentration in mol/dm3 × volume in dm3 moles of sodium carbonate = 0.05 x 0.25 = 0.0125 mol Step 2: Use the RFM of sodium carbonate to convert moles into grams. RFM of Na2CO3 = (2×23) + 12 + (3×16) = 106 1 mol = 106g 0.0125 mol = ? 106 x 0.0125 = 1.33g Ms. Christine Gatt Page 5 of 10 TITRATION Choice of Indicator TITRATION PROCEDURE Example: Titration of sodium carbonate standard solution against hydrochloric acid of unknown concentration. Precautions (STEP 1): Before starting the titration, it is important to: ✓ Wash the graduated or volumetric pipette with distilled water followed by the solution it is going to contain. ✓ Wash the conical flask with distilled water only. Ms. Christine Gatt Page 6 of 10 STEP 1: Transfer 25 cm3 of sodium carbonate solution to a clean conical flask using a volumetric pipette and pipette filler. STEP 2: Using a teat pipette, 2 drops of phenolphthalein indicator are added to the sodium carbonate solution in the conical flask. STEP 3: A clean burette is filled with dilute hydrochloric acid using a glass funnel. The initial volume of the solution in the burette is recorded. Precautions (STEP 3): ✓ Wash the burette with distilled water followed by the solution it is going to contain (in this case dilute hydrochloric acid) before this step is carried out. ✓ After filling the burette with the reagent solution, open the stopcock long enough to fill the burette tip. ✓ A dark or clear background can be placed behind the burette at the meniscus. This makes it easier to read the scale. Ms. Christine Gatt Page 7 of 10 STEP 4: The burette valve is open so that the hydrochloric acid solution is added to the conical flask, while the flask is swirled regularly, until the indicator changes colour from pink to colourless. The final volume is read and is recorded as a rough titre value. It gives an estimate of the volume of hydrochloric acid needed to fully neutralize the 25 cm3 of sodium carbonate solution in the conical flask. Precautions (STEP 4): STEP 5: Step 4 is repeated, this time adding the hydrochloric acid dropwise to accurately determine the exact volume needed. The experiment is repeated multiple times until at least two concordant titre values that do not vary by more than + 0.1 cm3 from each other are obtained. Precautions (STEP 5): ✓ When the solution needed has been delivered, close the stopcock and touch the inner wall of the flask to the burette tip to remove any hanging drop. STEP 6: The average titre value is calculated. The rough titre value is not included in this calculation. Calculations are then carried out to determine the concentration of hydrochloric acid. Table of results: Rough titration 1st Titration 2nd Titration Initial reading (cm3) Final reading (cm3) Titre value (cm3)* Average titre value (cm3) Titre value = Final reading – Initial reading Ms. Christine Gatt Page 8 of 10 Overview of the titration process Ms. Christine Gatt Page 9 of 10 Worked example In a titration, 25.0 cm3 of 0.100 mol/dm3 sodium hydroxide solution is exactly neutralised by 20.00 cm3 of a dilute solution of hydrochloric acid. Calculate the concentration of the hydrochloric acid solution. Step 1: Calculate the amount of sodium hydroxide in moles Volume of sodium hydroxide solution = 25.0 ÷ 1,000 = 0.0250 dm 3 Rearrange: Concentration in mol/dm3 = moles of solute volume in dm3 moles of solute = concentration in mol/dm3 × volume in dm3 moles of sodium hydroxide = 0.100 × 0.0250 = 0.00250 mol Step 2: Find the amount of hydrochloric acid in moles The balanced equation is: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) So the mole ratio NaOH:HCl is 1:1 Therefore 0.00250 mol of NaOH reacts with 0.00250 mol of HCl Step 3: Calculate the concentration of hydrochloric acid in mol/dm 3 Volume of hydrochloric acid = 20.00 ÷ 1000 = 0.0200 dm3 Concentration in mol/dm3 = moles of solute volume in dm3 Concentration in mol/dm3 = 0.00250 = 0.125 mol/dm3 0.0200 Ms. Christine Gatt Page 10 of 10