Khalifa University CHEM115 Lab 4: Acid-Base Titration PDF

Acid-Base Titration Lab 4 CHEM115 Lab Manual SUMMER 2024  Khalifa University College of Engineering and Physical Sciences Chemistry-CHEM 115 Labs Acid-Base Titration The objective of the Exper...

Acid-Base Titration Lab 4 CHEM115 Lab Manual SUMMER 2024  Khalifa University College of Engineering and Physical Sciences Chemistry-CHEM 115 Labs Acid-Base Titration The objective of the Experiment In this experiment, you will: Learn how to perform an acid-base titration Use titration to determine the molarity and percent by mass of acetic acid in a vinegar solution. Introduction and theory Titration is a quantitative, volumetric procedure used to determine the concentration of a solute in a solution. In Titration, a solution of accurately known concentration, called a standard solution, is added gradually to the unknown concentration solution under investigation to react exactly with all of the substance. When the reaction is complete, the titration has reached its ‘ End point’ or commonly agreed on as the ‘Equivalence point’. This point is often determined using an indicator. For example, when an acid reacts with a base, the indicator is a compound that has a different color in acidic or basic media. In this practical, you will use phenolphthalein as the indicator; it is colorless in acidic solutions but changes to pink in basic solutions. When the indicator changes color, the endpoint of the titration has been reached. The principle of Titration is to accurately measure the volume of the standard (known concentration) solution needed to react with the solution being investigated. Then, the concentration of the unknown solution can be determined. Two standard units for expressing the concentration of a solution are: Number of moles of solute Molarity = 1 M = 1 mol L-1 Volume of solvent in L 1 w% = 1 g (100g)-1 Mass of solute (g) Percentage by mass= 100% Mass of solution (g) 2  Khalifa University College of Engineering and Physical Sciences Chemistry-CHEM 115 Labs Acid-Base Titration Concentrations measured in mol L-1 are usually represented by [ ]; [0.25] M = 0.25 mol L-1. In this experiment, you will titrate a vinegar solution with a standardized sodium hydroxide, NaOH solution. Vinegar contains acetic acid, HCH3CO2, an organic monoprotic acid. The systematic name is ethanoic acid. You will calculate the molarity of acetic acid in the vinegar and the percentage by mass of acetic acid in the vinegar from your titration data. The reaction involved is: HCH3CO2 (aq) + NaOH (aq) → NaCH3CO2 (aq) + H2O (l) So, 1 mol of HCH3CO2 reacts with 1 mol of NaOH (aq). If the standard NaOH solution has a concentration of [cNaOH] mol L-1 and x mL are required to reach the endpoint, the number of moles of NaOH added is given by: Volume 𝑵𝒂𝑶𝑯 added (mL) (1) Number of moles of NaOH (mol)=𝑐NaOH molL-1 X 1000 (mL L−1 ) From the balanced equation, 1 mol of HCH3CO2 reacts with 1 mol of NaOH (aq), Ratio 1:1 this number must also be the number of moles of acid. The concentration of acid, cacid, is therefore given by: Number of moles of acid (mol)×1000 (mL L−1 ) cacid (mol L−1 ) = (2) Volume acid used (mL) Standard Solutions Solutions with an accurately known concentration are called standard solutions. Standard solutions of some substances can be prepared by dissolving a known mass of the substance in the correct amount of water to give an accurately known volume of solution or by accurately diluting a given volume of solution whose concentration is known. Sometimes, though, standard solutions cannot be prepared using these methods. The substance being weighed out may be of unknown purity or react with water, oxygen, or carbon dioxide in the atmosphere. The original concentration of a solution being diluted is not known accurately. For example, solid NaOH reacts with water and carbon dioxide present in the air, so the percentage purity cannot be accurately known. Dissolved gases, like ammonia NH3(g), are soluble in water. The exact amount of gas or volatile liquid, such as acetic acid, dissolved in water is difficult to determine. In these cases, approximate concentrations of the solutions are prepared and then titrated accurately to determine their concentration. 3  Khalifa University College of Engineering and Physical Sciences Chemistry-CHEM 115 Labs Acid-Base Titration Procedure Summary Take an accurately known volume of vinegar with an unknown concentration. Add an indicator and then add exactly enough standardized NaOH solution to react with all the acid. Knowing the concentration of the NaOH solution and the volume of both components, find the concentration of acid in the vinegar. Chemicals and materials are at the common front bench. ▪ Vinegar with a n unknown concentration is in a labeled capped glass vial. ▪ A standardized solution of sodium hydroxide is provided in a stoppered conical flask, ▪ Remember to record the concentration in your datasheet. ▪ Magnet Stirring bar in a glass vial and the Stirring bar retriever. Chemicals and Materials are in your tray ▪ Phenolphthalein in Dropper bottle ▪ Burette and Volumetric Pipette on the clamp fitted on the Retort Stand. ▪ Pipette filler ▪ Erlenmeyer Flask #3 ▪ Filter Funnel ▪ Hotplate stirrer Methods 1. Rinse your burette with distilled water. 2. Get a stoppered flask containing approx. 125 mL standard NaOH solution for titration. Record the concentration of the sodium hydroxide solution. (standard solution) 3. Rinse the burette twice with about 5 mL NaOH solution, and let the solution drain through the stopcock into a waste beaker. 4. Fill the burette to above the 0.00 mL mark with NaOH and use the funnel so no spills will occur. 5. Allow some of the solutions to run out into your waste beaker until the level in the burette is between the 0.00 mL and 5.00 mL marks. Check to see that there are no air bubbles between the tip of the burette and the stopcock. Record the exact initial volume of NaOH on your data sheet. 6. Rinse your 10 mL pipette with a small volume of vinegar and drain it into your waste beaker. 7. Use the 10 mL pipette to accurately transfer 10.00 mL of your unknown vinegar into a clean 250 mL Erlenmeyer flask. Be sure to use the pipette properly, as demonstrated at the beginning of the lab. Be careful that liquid is not drawn up into the pipette bulb.  Khalifa University College of Engineering and Physical Sciences Chemistry-CHEM 115 Labs Acid-Base Titration 4 8. Add about 25 mL of distilled water and 2–3 drops of phenolphthalein indicator to the vinegar solution and swirl. Carefully slide a stir bar down the side of the flask into the solution. Place the flask onto the middle of the stirrer hot plate and arrange the burette so that the tip is about 1 cm inside the neck of the flask. Adjust the stirrer control for a reasonable stirring rate (about the #3 setting). 9. Begin the titration by adding approximately 1.00 mL of the sodium hydroxide solution to the acid solution. Allow the solution to stir for a few seconds and observe the color change – if any. Continue adding 1 mL of NaOH solution until the solution turns pink and stays pink. Record the volume of NaOH solution added. Calculate the approximate endpoint volume. 10. Turn off the stirrer motor before lifting the flask away from the surface of the hotplate/stirrer. Use the stir bar retriever to remove the stir bar and pour the flask's contents down the sink. Do not pour the stir bar into the sink. 11. Repeat the above titration, but this time accurately for three trials. Use a clean Erlenmeyer flask with 10.00 mL of vinegar. You should not have to refill the NaOH-containing burette at this stage. You can add a volume of NaOH solution of 3 – 4 mL less than the approximate endpoint volume. Ensure the solution is stirred thoroughly, and then begin adding NaOH drop by drop. As the color takes longer to disappear, reduce the quantity of sodium hydroxide added. Eventually, as you get near the endpoint, you should be adding the sodium hydroxide a fraction of a drop at a time. Use your distilled water squirt bottle to wash the drops of sodium hydroxide into the Erlenmeyer flask. The endpoint is reached when the pink color stays for at least 30 seconds. Record the endpoint volume. 12. Repeat Step 10. You need to obtain at least three accurate titrations on 10.00 mL portions of the vinegar solution where the endpoints agree closely If you have performed the above steps correctly, you have enough vinegar and NaOH solution for one approximate titration and at least three accurate titrations. If you have time, you can perform additional accurate titrations. Cleanup - Pour your waste rinses and any extra vinegar and sodium hydroxide solution into the large waste beaker next to your tray on the bench. - Clean all your glassware, including the burette. Put the clean burette upside down in the burette clamp. - Return the capped Erlenmeyer flask and the vinegar glass vial to the front bench. - Make sure to dry the Magnet Stirring bar and place it in the glass vial. - Fill the distilled water bottle. - Leave your work area clean and tidy. 5  Khalifa University College of Engineering and Physical Sciences Chemistry-CHEM 115 Labs Acid-Base Titration Laboratory Risk Assessment Sheet Hazard Codes Reagents, products, and instruments T A F I O W Other NaOH (< 0.5 M) 1 Burette 1 Pipette 2 Pungent Vinegar (Commercials ≈ 6%) aroma smell Indicator solution Phenolphthalein 0.5% 2 T Toxic A Corrosive/Irritant F Flammable I Instrument, glassware, or equipment hazard O Oxidizing agent W Reacts with water C Carcinogenic G Liberates toxic gases on contact with water/acids or bases E Explosive M Microbiology hazard N Environmental hazard R Radioactive Hazard Code Numbers Hazard Code Hazard level number Nonsignificant 0 Low 1 Medium 2 High 3 Severe 4 Extreme 5 6  Khalifa University College of Engineering and Physical Sciences Chemistry-CHEM 115 Labs Acid-Base Titration Exposure Potential Codes Exposure potential Letter Code Low X Medium XX High XXX Protection Code Safe Highest Exposure Risk operating hazard Preparation of unknown 1 vinegar solution Pipette I2 T2 X P1 and adding phph 2 Titration Burette A1 I1 X P1 P1 Open laboratory (lab coat and safety glasses required). P2 Restricted open laboratory (no naked flames allowed). P3 Contact protection is required (lab coat, safety glasses, and suitable gloves). P4 Fume cupboard required. P5 Fume cupboard and additional safety equipment required (gloves, visor/ respirator, etc.). P6 Glove box/ remote operation required. 7