Experiment 6 Acids and Bases PURPOSE The purpose of this experiment is to study the properties of acidic/basic substances using indicators and a pH meter. INTRODUCTION Acids and bases undergo complete or incomplete ionization when dissolved in water and are called strong acids, strong bases, weak acids or weak bases. The experimental determination of the pH of a solution is commonly performed either by the use of indicators or pH meter. The acid or base dissociation (ionization equilibrium) constant, Ka or Kb can be determined experimentally. A sample of a weak acid (HA) is dissolved in water and then divided into two equal-volume portions. When one portion is titrated with a sodium hydroxide solution, all HA molecules present are converted into Aions: OH + HA→> H,O A Ka [H][A][HA] = [H]=10 PH The number of A ion produced is equal to the number of moles of HA in the original half-portion and is also equal to the number of moles of HA in the unused portion of a weak acid. The value of Ka can be determined by measuring the pH of a half-neutralized sample of the acid. When the acid-base titration is carried out, the endpoint of the titration can be determined when the indicator changes colour. The change of pH can be measured using a pH meter. Exploring Chemistry Laboratory Experiments for General Chemistry 2nd Edition DATASHEET EXPERIMENT 6: ACIDS AND BASES Name: Date: Student ID: Group: A. pH using Indicator Table 6.1: pH using Indicator Indicator Colour change pH of Colour change colour in Sample 1 Colour change in Sample 2 change Thymol blue Red-yellow 1.2-2.8 Yellow-blue 2.8-9.6 Pink Blue Bromophenol blue Yellow-blue 3.0-4.7 Yellow Blue Methyl orange Red-yellow 3.2-4.4 Reddish Yellow B. Determination of K for a Weak Acid 24 ml NaOH Determination of Ka for a weak acid: pH of half neutralized solution of unknown weak acid: 5.1pH C. Strong Acid-Strong Base Titration Table 6.2: Strong Acid-Strong Base Titration a Total HCl added (mL) 0 10 15 20 23 25 26 26 27 30 30 PH 7 should b Measured pH be 12.08 11.15 10.84 10.10 9.80 6.51 4.48 3.70 2.53 NaOH+HCI NaCl + H₂O 25ml 0.2 M M M₂ V₁ V₂ 0.2M 0.2M 25 ml V₂ 0.2M neutral V₂-25mL 28 28 Exploring Chemistry Laboratory Experiments for General Chemistry 2nd Edition CHEMICALS AND APPARATUS 0.2 M Sample solution A 0.2 M Sample solution B Unknown acid solution Test tubes Conical flask Beaker Indicators (thymol blue, bromophenol blue, methyl orange and Pipet (25 mL) phenolphthalein) Burette 1-2 drops Telicator pH meter PROCEDURE Sample solution 1 HCI Y Y Y ~ - 1-2 mL A. pH using Indicator 1. 2. 3. 4. Repeat steps 1-3 for other sample solutions. sample solution 2 = NaOH Y Y Y Place about 1-2 mL of the sample solution in small test tubes and add 1-2 drops of one of the indicators given in Table 6.1. Record the colour of the solution after adding the indicator in Table 6.1. Repeat steps 1-2 for other indicators. B. Determination of K for a Weak Acid 1. 3. Fill the burette with 0.2 M NaOH solution. Pipette 25 mL of an unknown 0.2 M weak acid solution into a conical flask, add 3-4 drops of phenolphthalein indicator and titrate the acid solution until a faint persistent pink colour appears. Add 25 mL of the same unknown 0.2 M weak acid solution into the neutralized solution (Step 2). Stir well. Record the pH of the solution. C. Strong Acid-Strong Base Titration 1. Fill the burette with 0.2 M HCI. 12 34 2. 3. 4. 5. 6. Using a pipet, transfer 25 mL of 0.2 M NaOH into the 100 mL beaker and add 3-5 drops of phenolphthalein. Calibrate a pH meter at pH 7 and pH 4. Measure the pH of NaOH and record in Table 6.2. The tip of the pH meter should be submerged for the entire experiment. Carefully add 10 mL of HCI (burette) into the NaOH solution. Mix solution gently and measure the pH. Continue adding acid and measuring pH as indicated in Table 6.2 of the datasheet. 26 26