Acid Base Titration Experiment ACID - BASE TITRATION LAB

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1 ACID - BASE TITRATION LAB MATERIALS and CHEMICALS Burette 50 ml Burette clamp Ring stand Stirring rod Plastic funnel Beakers (50 ml, 100 ml, 400 ml) Graduated cylinder (25 ml, 50 ml) 0.10 M NaOH 0.10 M HC 2 H 3 O M HCl Phenolphthalein NOTE Acid and Base concentrations of 1.0 M or 2.0 M can be used if available. Purpose 1. To demonstrate the basic laboratory technique of titration 2. To determined the concentration of acid and base solution through titration with standard solution. 3. To carry out acid base titration using phenolphthalein as indicator. INTRODUCTION Acid base titration involves a neutralization reaction in which an acid is react with an equivalent amount of base. For the neutralization of acid with sodium hydroxide: NaOH + HA NaA + H 2 O In this lab you will investigate the qualitative and quantitative aspects of acid-base reactions, also known as neutralization reactions. Using known concentrations of acid OR base in a neutralization reaction allows for later determination of unknown molarities and/or volumes of the counterpart base or acid in the reaction. Experimentally, this technique involves the process of titration (a titer refers to a known or fixed volume). Thus we will be adding a fixed volume and concentration of base to an acid to find the acid concentration. Neutralization occurs when acid and bases exist in comparable stoichiometry, for instance the amount of the acid (mole) is equivalent with the amount of base (mole). The endpoint of titration can be determined using indicator. To determine when neutralization occurs, an indicator such as phenolphthalein can be used. An indicator is a substance which undergoes a distinct color change at or near the equivalence point. The point at which the indicator changes color and the titration is stopped is called the endpoint. Ideally, the endpoint should coincide with the equivalence point. Phenolphthalein is colorless in acidic solution and reddish violet (pink) in basic solution. Molarity or molar concentration is a common unit for expressing the concentration of solutions. It is defined as the number of moles of solute per liter of solution (or millimoles of solute per milliliter of solution). Normality is number of mole equivalents per liter of solution. Morality is used for total volume of solution, whereas normality is used for acids or bases. If you know the Molarity of an acid or base solution, you can easily convert it to Normality by multiplying Molarity by the number of hydrogen (or hydroxide) ions in the acid (or base). For example, a 2 M H 2 SO 4 solution will have a Normality of 4N (2 M x 2 hydrogen ions). The concentration of a basic solution can be determined by titrating it with a volume of a standard acid solution (of known concentration) required to neutralize it. 1

2 The purpose of the titration is the detection of the equivalence point, the point at which chemically equivalent amounts of the reactants have been mixed. The amount of reactants that have been mixed at the equivalence point depends on the stoichiometry of the reaction. Acid-Base Titration An acid-base titration is a neutralization reaction that is performed in the lab in order to determine an unknown concentration (Molarity) of acid or base. As long as the concentration of one of the solutions is known, the concentration of the other reaction can be obtained through titration. In a titration reaction, the moles of acid equal the moles of base at the equivalence point. Once the equivalence point is reached the concentration of the unknown solution can be determined because: Normality Equation Molarity Equation N a V a = N b V b n a M a V a = n b M b V b Where, n a = number of hydrogen in acid M a = molarity of acid V a = molarity of base nb= number of hydroxide in bas M b = molarity of base V b = volume of base N a = normality of acid N b = normality of base Example 1 - The titration of ml of M HCl requires ml of a NaOH solution. To find the molarity (molar concentration) of the NaOH solution: HCl + NaOH NaCl + H 2 O M a V a = M b V b (0.184 moles/l HCl) (0.016 L) = M (NaOH) (0.025 L) M (NaOH) = M Example 2 - The titration of ml of 0.50 M H 2 SO 4 requires ml of a NaOH solution. To find the molarity (molar concentration) of the NaOH solution: H 2 SO NaOH Na 2 SO 4 + 2H 2 O n a M a V a = n b M b V b (2)(0.5 moles/l H 2 SO 4 ) (0.020 L) = (1) (M b ) (0.025 L) M (NaOH) = 0.8 M Example 3 How many moles of KOH required for titration of 40.0 ml, 2.0 M HCl solution? n a M a V a = n b M b V b Moles KOH (M b V b ) = (n a M a V a )/n b (1)(2.0)(40.0/1000) / (1) = 0.08 moles KOH Safety Precautions Safety goggles must be worn at all times. The acid solutions are dilute acid but can harm eyes, skin, and clothing. Handle with care. Any acid spilled on the skin or splashed into your eyes should be rinsed with a large volume of water. NaOH solutions are corrosive to the skin and can harm your eyes. If spilled on the skin or splashed into your eyes, flush with a high volume of water. Disposal - The acid solutions, base solution, and neutralized mixture may all go into the sink. 2

3 PROCEDURE In this experiment, you will determine the molarity of the NaOH solution which has been previously prepared. You will do this by using the NaOH solution as a titrant and titrate standard acid solution until you reach the endpoint. Two standard acid solutions (HCl and CH 3 COOH) of 0.10 M will be available. The volumes of NaOH required to neutralize the acid solution will be tabulated and compared. Rinse the 50 ml burette three times with deionized water, and then twice with about 5 ml of the sodium hydroxide solution. Clamp the burette in position and fill the burette to just above the zero mark with the basic solution. Be sure to let any bubbles out of the tip of the burette. Bring the NaOH solution meniscus down to or below the zero mark, read, and record the burette reading. Fig.1 Fig. 1 Use a graduated cylinder to add 20.0 ml portions of the standard acid solution to each of three clean Erlenmeyer flasks. Add 2-3 drops of phenolphthalein to each flask. Set one flask under the burette, and add sodium hydroxide slowly, swirling the flask constantly. As you approach the endpoint, a light pink, indicating a local excess of the titrant, NaOH. Slow down the rate of addition of sodium hydroxide when a pink color begins to persist momentarily outside the place where the solutions mix. Proceed dropwise from this point. Before the endpoint is reached, a temporary pink color will be seen where the NaOH drop hits the bulk solution. Stop as soon as the first permanent pink color appears. This is the endpoint. Read the burette and record the reading. Repeat with the other two flasks. Calculate the molarity of the NaOH solution separately for each titration. NOTE: Always refill your standard NaOH burette with funnel before starting a new titration 3

4 ACID - BASE TITRATION T XXE Name Date: Instructor Part I Titration of Strong Acid, HCl and Strong Base, NaOH Write the balanced equation to represent this neutralization reaction. Table 1 Data Volume of acid used Molarity of acid used Initial burette reading Final burette reading Volume of NaOH used Molarity of NaOH Show Calculation Trial 1 Trial 2 Trial 3 AVERAGE MOLARITY OF NaOH M (Show calculation) 4

5 Part II Titration of Weak Acid, CH 3 COOH and Strong Base, NaOH Write the balanced equation to represent this neutralization reaction. Table 2 Data Volume of acid used Molarity of acid used Initial burette reading Final burette reading Volume of NaOH used Molarity of NaOH Show Calculation Trial 1 Trial 2 Trial 3 AVERAGE MOLARITY OF NaOH M (Show calculation) 5

6 ACID - BASE TITRATION T XXE Name Date: Pre -Laboratory Review Questions and Exercises DUE BEFORE LAB BEGINS. ANSWER IN THE SPACE PROVIDED 1. Define the following terms. a. Blank titration- b. Back titration - 2. If it takes 54 ml of 0.10 M NaOH to neutralize 125 ml of an HCl solution, what is the concentration of the HCl? 3. What is titration used for in chemistry? 4. The container, flask, you obtain the acid solution must be dry. Why? 5. If it takes 25.8 ml of 0.05 M H 2 SO 4 to neutralize 32.5 ml of NaOH solution, what is the concentration Of the NaOH solution? 6

7 ACID - BASE TITRATION T XXE Name Date: Post -Laboratory Review Questions and Exercises DUE AFTER COMPLETING LAB. ANSWER IN THE SPACE PROVIDED 1. Define the following terms. Titrant Analyte 2. Why should you titrate into an Erlenmeyer flask rather than into a beaker? 3. Calculate the normality of a solution containing 0.60 mol of H 2 SO 4 in 400 ml of solution. 4. Why should you NOT plan to start the titration with the acid and base burettes filled exactly to the zero mark? 5. Titration reveals that 11.6 ml of 3.0 M sulfuric acid are required to neutralize the sodium hydroxide solution. Calculate; a) number of moles of NaOH in solution. b) grams mass of sodium hydroxide in solution. 7