酸碱滴定实验

1、ACID BASE TITRATION CHEMISTRY LABINTRODUCTION: In the chemistry laboratory, it is sometimes necessary to experimentally determine the concentration of an unknown acid or base solution. A procedure for making this kind of determination is called an acid-base titration. In this procedure, a solution o

2、f known concentration, called the standard solution, is used to neutralize a precisely measure volume of the solution of unknown concentration, to which one or two drops of an appropriate acids-base indicator have been added. If the solution of unknown concentration is acidic, a standard base soluti

3、on is added to the acid solution until it is neutralized. If the solution of unknown concentration is basic, a standard acid solution is added to the base solution until it is neutralized.While carrying out an acid-base titration, you must be able to recognize when to stop adding the standard soluti

4、on, that is, when neutralization is reached. This is the purpose of the acid-base indicator mentioned above. A sudden change in color of the indicator signals that neutralization has occurred. At this point, the number of hydronium ions from the acid equals the number of hydroxide ions from the base

5、. The point at which this occurs is called the endpoint of the titration. When the endpoint is reached, the volume of the standard solution is carefully determined. Then, the measured volumes of the two solutions and the known concentration of the standard solution can be used to calculate the conce

6、ntration of the unknown solution. The following steps tell us how to calculate the unknown concentration:1. Write the balanced equation for the reaction: Acid + Base Salt + HOH From the coefficients, determine how many moles of acid react with moles of base2. The following relationship can be used t

7、o calculate the unknown concentration (note: the coefficients of acid & base come from the balanced equation)(H+) x MA x VA= (OH-) x MB x VBWhere MA = molarity of acid solutionMB = molarity of base solutionVA = volume of acid solutionVB = volume of base solutionH+ = moles of hydronium ions in the ac

8、idOH- = moles of hydroxide ions in the baseExample #1: Step 1: H2SO4 + 2 NaOH Na2SO4 + 2 HOH Step 2: (2) x MA x VA = (1) x MB x VBPURPOSE: To determine the molarity of a NaOH solution by titrating it with a standard acid solutionEQUIPMENT:50 mL buret2 Erlenmeyer flasks, 150 mLfunnelRing standdropper

9、 pipetBuret clampgraduated cylinderGoggles2 beakers (any size)MATERIALS:Standard Acid Solution: _M HCldistilled waterNaOH (concentration unknown)indicator solution (bromthymol blue)PROCEDURE:One partner does steps 1 3 while the other partner does steps 4-7.1. Rinse the buret with tap water. Clamp it

10、 to a ring stand with the buret clamp.2. Obtain about 30 mL of base solution in a clean, dry 150 mL beaker. CAUTION: Handle these solutions with care. They can cause painful burns if they contact the skin.3. Fill the buret with the basic solution (use a funnel the pour the base into the buret). Plac

11、e the beaker which held the base in it under the tip of the buret. Then allow the base to flow out the buret tip until the bottom of the meniscus is at the nearest whole number mL mark (SEE FIGURE 1). This will be the initial reading of base in trial 1, record this volume in the data table. figure 1

12、The volume should be recorded to the nearest 0.01 mL.4. Obtain about 30 mL of acid solution in a clean, dry 150 mL beaker.5. Measure out exactly 10.0 mL of HCL into a graduated cylinder (pour less than 10 mL into the cylinder and finish to 10.0 mL with a dropper pipet). Pour the acid into a 125 mL E

13、rlenmeyer flask.6. Add exactly 10.0 mL of distilled water to the Erlenmeyer flask (measured the same way as the acid). ADD 3 DROPS OF INDICATOR TO THE FLASK. Swirl the flask to mix all the ingredients. Record the volume of acid used in trial 1 in the data table. The volume should be recorded to the

14、nearest 0.1 mL.7. Repeat steps 4-6 for a second flask. Record this volume of acid in trial 2 of the data table. When you are done, you should have two flasks containing acid and indicator.8. Place a sheet of white paper under the buret (makes it easier to see the color change for the indicator). Pla

15、ce the flask under the tip of the buret. To avoid splashing, be sure the tip of the buret is in the flask as illustrated to the right.9. Swirling the flask gently, begin the titration by adding base to the flask drop by drop. Continue until a faint color remains for about 30 seconds. If over-titrati

16、on occurs (the color is too deep), follow your teachers instruction for correcting this condition.10. Note and record the exact final volume reading on the scale of the buret. Discard the solution in the flask as per instructions. Wash and rinse the flask before putting it away.11. Repeat the titrat

17、ion (steps 8-10). IT IS NOT NECESSARY TO REFILL THE BURET BETWEEN TRIALS, simply read and record the initial volume of the solution in the buret and place that value in the data table under trial 2 initial reading buret. If time permits do a third trial. Name _ Acid-Base TitrationDate _Write up Shee

18、tPeriod/Section _A. PURPOSE:B. PRE-LAB QUESTIONS:Define the following: a. Titration: _ _ _b. Standard solution: _ _ _c. Endpoint: _ _ _C. OBSERVATIONS:1. Color of indicator in acidic solution_ Color of indicator in neutral solution _ Color of indicator in basic solution _2. _D. DATATrial 1Trial 2Tri

19、al 3BaseBaseBaseInitial reading buretFinal reading buretVolume Base used Volume Acid usedE. CHEMICAL REACTION (Balanced Neutralization Reaction):_ + _ _ + _F. CALCULATIONS: show equation used, insert values, and give answer with appropriate unitsFor each trial, calculate the molarity of the base solution (unknown concentration). TRIAL #1