Titration and Buffers

Chemistry Department

Minneapolis Community & Technical College

Intro to Chemistry Chem1020 Lab

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Overview• Part IIntroduction

• Part IINaOH in the flask

• Part IIIHCl in the buret

• Part IVTitration

• Part VCalculation

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Part I. Introduction• This experiment is about acid-base reactions in which a proton (H+) is

transferred from one reactant to the other :

HA + B- A- + HB

• Your instructor will demonstrate the part involving a buffer solution. Buffer solutions can resist change in their pH even when a small amount of acid or base is added. This part is discussed in Chapter 14.10 in the textbook.

• You will perform the part involving titration, a process in which an HCl solution of unknown concentration is slowly added to a given amount of NaOH solution with known concentration till they react completely with each other.

• After the titration, you will determine the concentration of the HCl solution based on other information. This part is discussed in Chapter 14.6 in the textbook.

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Part II. NaOH Solution in the Flask

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2. Pour ~30mL into a beaker labeled as “Base”

1. Record this concentration of NaOH solution.

3. Use a medicine dropper to transfer 10.00 mL into a graduated cylinder.

4. Record this volume of NaOH solution.

5. Pour it into an Erlenmeyer flask.

6. Add 2-3 drops of thymol blue.

Part III. HCl Solution in the Buret

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Covering beaker Remove before starting the experiment, and put it back when all done.

BuretMake sure it is secured

by the buret clamp.

Waste beaker It is used to collected all waste solutions. Empty into sink when all done.

Buret clamp

ValveNow it is in the “OFF” position.

This buret has an incre-ment of 0.1 mL (why?), so all readings should go to the hundredth place with 2 decimal places.

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Part III. HCl Solution in the Buret

1. Pour ~30mL into a beaker labeled as “Acid”

2. Fill the buret with the help of a funnel to around the “0 mL” mark.

3. Check the buret bottom for bubbles.

4. Open the valve to flush the bubbles into the “waste beaker”.

Remove the funnel when done!!!

Part IV. Titration

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1. Read the bottom of the meniscus in the buret and record it as the “Initial Reading”. E.g. the above volume should be recorded as 0.80 mL.

2. Put the NaOH solution under the buret and turn on the valve. Keep swirling the flask gently.

3. Close the valve immediately after the solution turns yellow which indicates the completion of the reaction.

4. Read the bottom of the meniscus again and record it as the “Final Reading”. This one should be recorded as 20.41 mL.

Make sure to read the buret both before and after the titration!!!

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.1. Empty the content in the Erlenmeyer flask into sink. Rinse the

flask first with tap water followed by distilled water. No drying is needed.

2. Repeat steps 3-6 in slide #4 to get NaOH solution ready in the Erlenmeyer flask.

3. Repeat steps 2-4 in slide #6 to get HCl solution ready in the buret.

4. Repeat all steps in slide #7 for titration. To get a more accurate result this time, slow down by adding HCl dropwise when close to the endpoint.

5. Empty the contents in beakers and flask into sink. Rinse first with tap water followed by distilled water. Return everything to your bench.

Part IV. Second Trial

Part V. Sample Calculation

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NaOH SolutionMolarity (M) 0.12 M

Volume taken into flask 10.00 mL

Final reading in buret (mL) 20.41 mL

Initial reading in buret (mL) 0.80 mL

Volume used (VHCl) (mL) (20.41 - 0.80) = 19.61 mL (2 decimal places)

HCl Solution

Moles of NaOH0.12 M × 10.00 mL × 1L / 1000 mL= 0.0012 mole NaOH (2 sig. figs.)

Moles of HCl (= Moles of NaOH) 0.0012 mole HCl (2 sig. figs.)

Molarity of HCl 0.0012 mole HCl 1000mL——————— × ———— = 0.061 M (2 sig. figs.) 19.61 mL 1 L

)mL1000

L1VM( NaOHNaOH

)L1

mL1000

V

HCl of Moles(

HCl